- Add semaphore alloc/free/acquire/release functionality to libgolang runtime;
- Implement semaphores for thread and gevent runtimes.

  * Thread runtime uses PyThread_acquire_lock/PyThread_release_lock +
    PyThread_acquire_lock/PyThread_release_lock, which, if used
    carefully, do not depend on GIL and on e.g. POSIX are tiny wrappers
    around sem_init(process-private) + sem_post/sem_wait(*).

  * Gevent runtime uses geven't Semaphore in Pyx mode.

- Add Sema and Mutex classes that use semaphores provided by a runtime
  in a RAII style.
- Add with_lock(mu) that mimics `with mu` in Python.

Sema and Mutex will be used in channels implementation in the followup
patch.

(*) during late testing a bug was found in CPython2 and PyPy semaphore
implementations on Darwin (technically speaking on POSIX with
_POSIX_SEMAPHORES undefined). Quoting the patch:

    FIXME On Darwin, even though this is considered as POSIX, Python uses
    mutex+condition variable to implement its lock, and, as of 20190828, Py2.7
    implementation, even though similar issue was fixed for Py3 in 2012, contains
    synchronization bug: the condition is signalled after mutex unlock while the
    correct protocol is to signal condition from under mutex:

      https://github.com/python/cpython/blob/v2.7.16-127-g0229b56d8c0/Python/thread_pthread.h#L486-L506
      https://github.com/python/cpython/commit/187aa545165d (py3 fix)

    PyPy has the same bug for both pypy2 and pypy3:

      https://bitbucket.org/pypy/pypy/src/578667b3fef9/rpython/translator/c/src/thread_pthread.c#lines-443:465
      https://bitbucket.org/pypy/pypy/src/5b42890d48c3/rpython/translator/c/src/thread_pthread.c#lines-443:465

    This way when Pygolang is used with buggy Python/darwin, the bug leads to
    frequently appearing deadlocks, while e.g. CPython3/darwin works ok.

    -> TODO maintain our own semaphore code.

So eventually we'll have push down and maintain our own semaphores,
at least for platforms we care, not to be beaten by CPython runtime bugs.

Move `ch = py{send,recv}.__self__` to right after case __class__ check.
For now both versions - old and new - can work, but when we'll move
channel implementations to C the new version will be required to access
pychan C-level attribute earlier than where ch is currently initialized.

For clarity to denote that things work at Python level:

    - casev	-> pycasev
    - case	-> pycase
    - recv	-> pyrecv
    - send	-> pysend

Channel object is still denoted as `ch` to reduce noise for when chan IO
code will be move into libgolang. `ch` will be renamed to `pych` after
that.

- put the logic to test-tweak what happens inside _blockforever into
  context manager pypanicWhenBlocked;
- place this manager in pyx code, where it can later be changed to tweak
  _blockforever at C level.

For clarity to distinguish where an object is Python-level channel, or
(later) a C-level channel.

Change `from golang import chan` to `from golang cimport pychan`; add
type annotations where pychan is used. Using pychan at C level will be
needed when test code will need to access C-level pychan attributes.

We will need to add C-level attributes to pychan and this requires it to
become cdef class. The class is exported because at least
_golang_test.pyx will also need to have access to those attributes.

If we just do `class pychan` -> `cdef class pychan` e.g. the following
starts to break:

    1.venv/local/lib/python2.7/site-packages/py/_path/local.py:701: in pyimport
        __import__(modname)
    golang/__init__.py:174: in <module>
        from ._golang import    \
    golang/_golang.pyx:455: in init golang._golang
        _pychan_send  = _pychan_send.__func__
    E   AttributeError: 'method_descriptor' object has no attribute '__func__'

and

    golang/_golang.pyx:513: in golang._golang.pyselect
        if im_class(recv) is not pychan:
    _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

    f = <built-in method recv of golang._golang.pychan object at 0x7f7055e57cc8>

        def im_class(f):
    >       return f.im_class
    E       AttributeError: 'builtin_function_or_method' object has no attribute 'im_class'

    golang/_pycompat.py:28: AttributeError

This is probably because for `cdef class` methods Cython does not
emulate full method bindings the same way as Python does.  Anyway we can
check which method is passed to pyselect by chanop.__name__ or by
inspecting PyCFunction directly. And not having method binding wrapper
should only remove a bit of overhead.

So we are ok with reworking send/recv chanop detection, and since
this way im_class provided by golang._pycompat becomes unused, it is
also removed.

The timings are probably within noise:

 (on i7@2.6GHz)

thread runtime:

    name             old time/op  new time/op  delta
    go               21.7µs ± 1%  20.0µs ± 1%  -7.60%  (p=0.000 n=10+10)
    chan             9.91µs ± 4%  9.37µs ± 4%  -5.39%  (p=0.000 n=10+10)
    select           19.2µs ± 4%  20.2µs ± 4%  +5.62%  (p=0.001 n=9+8)
    def              58.0ns ± 0%  58.0ns ± 0%    ~     (all equal)
    func_def         44.4µs ± 0%  43.8µs ± 1%  -1.22%  (p=0.000 n=10+10)
    call             63.0ns ± 0%  62.4ns ± 1%  -0.95%  (p=0.011 n=10+10)
    func_call        1.05µs ± 1%  1.06µs ± 1%    ~     (p=0.059 n=10+10)
    try_finally       135ns ± 0%   136ns ± 0%  +0.74%  (p=0.000 n=10+9)
    defer            2.36µs ± 1%  2.28µs ± 1%  -3.59%  (p=0.000 n=10+10)
    workgroup_empty  49.0µs ± 1%  48.2µs ± 1%  -1.63%  (p=0.000 n=10+9)
    workgroup_raise  62.6µs ± 1%  58.9µs ± 1%  -5.96%  (p=0.000 n=10+10)

gevent runtime:

    name             old time/op  new time/op  delta
    go               21.7µs ± 1%  20.5µs ± 1%  -5.33%  (p=0.000 n=10+9)
    chan             9.91µs ± 4%  9.72µs ± 5%    ~     (p=0.190 n=10+10)
    select           19.2µs ± 4%  19.5µs ±14%    ~     (p=0.968 n=9+10)
    def              58.0ns ± 0%  58.0ns ± 0%    ~     (all equal)
    func_def         44.4µs ± 0%  45.4µs ± 1%  +2.23%  (p=0.000 n=10+10)
    call             63.0ns ± 0%  64.0ns ± 0%  +1.59%  (p=0.000 n=10+10)
    func_call        1.05µs ± 1%  1.06µs ± 0%  +0.65%  (p=0.002 n=10+10)
    try_finally       135ns ± 0%   137ns ± 0%  +1.48%  (p=0.000 n=10+10)
    defer            2.36µs ± 1%  2.38µs ± 1%  +0.72%  (p=0.006 n=10+10)
    workgroup_empty  49.0µs ± 1%  48.2µs ± 1%  -1.65%  (p=0.000 n=10+10)
    workgroup_raise  62.6µs ± 1%  60.3µs ± 1%  -3.69%  (p=0.000 n=10+10)

For consistency to denote that this functions work at Python level:

    - len_sendq    -> pylen_sendq
    - len_recvq    -> pylen_recvq
    - waitBlocked  -> pywaitBlocked

Plain test code movement with s/panic/pypanic/ as in golang.pyx panic is
already there and means semantically different thing. We need this code to
live in pyx world, because when channels implementation will be moved to
C, this utilities will need to be adjusted in a way that is not possible
to do from Python.

Denote what a method returns via `# -> ...` suffix.

Use `ch` instead of `self` for pychan methods. This aligns with how
(py)chan objects are currently denoted in (py)select and will reduce the
difference when chan code is moved into libgolang. We are sticking to Go
convention here.

After the code with channels implementation is moved into libgolang, the
rest bits in golang.pyx will be changed to refer to pychan objects as
pych for clarity.

To denote that this function/classes work at Python level:

    - chan    -> pychan
    - select  -> pyselect
    - default -> pydefault
    - nilchan -> pynilchan

Plain code movement with just s/panic/pypanic/ as in golang.pyx panic is
aleady there and means semantically different thing. Moved code, even
though it lives in golang.pyx, is still Python code and requires Python
runtime and GIL. We'll be splitting channels implementation into nogil
world in the following patches.

Just plain movement to Cython brings the following speedup:

 (on i7@2.6GHz)

thread runtime:

    name             old time/op  new time/op  delta
    go               26.6µs ± 1%  21.7µs ± 1%  -18.54%  (p=0.000 n=10+10)
    chan             13.7µs ± 1%   9.9µs ± 4%  -27.80%  (p=0.000 n=10+10)
    select           29.3µs ± 2%  19.2µs ± 4%  -34.65%  (p=0.000 n=9+9)
    def              55.0ns ± 0%  58.0ns ± 0%   +5.45%  (p=0.000 n=10+10)
    func_def         44.0µs ± 1%  44.4µs ± 0%   +0.72%  (p=0.002 n=10+10)
    call             64.0ns ± 0%  63.0ns ± 0%   -1.56%  (p=0.002 n=8+10)
    func_call        1.09µs ± 1%  1.05µs ± 1%   -2.96%  (p=0.000 n=10+10)
    try_finally       139ns ± 2%   135ns ± 0%   -2.60%  (p=0.000 n=10+10)
    defer            2.36µs ± 1%  2.36µs ± 1%     ~     (p=0.617 n=10+10)
    workgroup_empty  58.1µs ± 1%  49.0µs ± 1%  -15.61%  (p=0.000 n=10+10)
    workgroup_raise  72.7µs ± 1%  62.6µs ± 1%  -13.88%  (p=0.000 n=10+10)

gevent runtime:

    name             old time/op  new time/op  delta
    go               28.6µs ± 0%  25.4µs ± 0%  -11.20%  (p=0.000 n=8+9)
    chan             15.8µs ± 1%  12.2µs ± 1%  -22.62%  (p=0.000 n=10+10)
    select           33.1µs ± 1%  23.3µs ± 2%  -29.60%  (p=0.000 n=10+10)
    def              55.0ns ± 0%  56.0ns ± 0%   +1.82%  (p=0.000 n=10+10)
    func_def         44.4µs ± 2%  43.0µs ± 1%   -3.06%  (p=0.000 n=10+9)
    call             64.0ns ± 2%  69.0ns ± 0%   +7.81%  (p=0.000 n=10+10)
    func_call        1.06µs ± 0%  1.06µs ± 1%     ~     (p=0.913 n=8+9)
    try_finally       136ns ± 0%   139ns ± 0%   +2.21%  (p=0.000 n=9+10)
    defer            2.29µs ± 1%  2.38µs ± 2%   +3.58%  (p=0.000 n=10+10)
    workgroup_empty  73.8µs ± 1%  70.5µs ± 1%   -4.48%  (p=0.000 n=10+10)
    workgroup_raise  94.1µs ± 0%  90.6µs ± 0%   -3.69%  (p=0.000 n=10+10)

- Add go functionality to libgolang runtime;
- Implement go for thread and gevent runtimes.

  * Thread runtime uses PyThread_start_new_thread which, if used
    carefully, does not depend on Python GIL and on e.g. POSIX reduces to
    tiny wrapper around pthread_create.

  * Gevent runtime uses gevent's Greenlet in Pyx mode. This turns
    gevent to be a build-time dependency.

- Provide low-level _taskgo in C client API;
- Provide type-safe C++-level go wrapper over _taskgo;
- Switch golang.go from py implementation into Pyx wrapper over
  Pyx/nogil API.

This is the first patch that adds Pyx/C++/C-level unit tests and hooks
them into golang_test.py .

This patch brings the following speedup to Python-level interface:

 (on i7@2.6GHz)

thread runtime:

    name             old time/op  new time/op  delta
    go               93.0µs ± 1%  26.6µs ± 1%  -71.41%  (p=0.000 n=10+10)
    chan             13.6µs ± 2%  13.7µs ± 1%     ~     (p=0.280 n=10+10)
    select           29.9µs ± 4%  29.3µs ± 2%   -1.89%  (p=0.017 n=10+9)
    def              61.0ns ± 0%  55.0ns ± 0%   -9.84%  (p=0.000 n=10+10)
    func_def         43.8µs ± 1%  44.0µs ± 1%   +0.66%  (p=0.006 n=10+10)
    call             62.5ns ± 1%  64.0ns ± 0%   +2.40%  (p=0.000 n=10+8)
    func_call        1.06µs ± 1%  1.09µs ± 1%   +2.72%  (p=0.000 n=10+10)
    try_finally       137ns ± 0%   139ns ± 2%   +1.17%  (p=0.033 n=10+10)
    defer            2.34µs ± 1%  2.36µs ± 1%   +0.84%  (p=0.015 n=10+10)
    workgroup_empty  96.1µs ± 1%  58.1µs ± 1%  -39.55%  (p=0.000 n=10+10)
    workgroup_raise   135µs ± 1%    73µs ± 1%  -45.97%  (p=0.000 n=10+10)

gevent runtime:

    name             old time/op  new time/op  delta
    go               68.8µs ± 1%  28.6µs ± 0%  -58.47%  (p=0.000 n=10+8)
    chan             14.8µs ± 1%  15.8µs ± 1%   +6.19%  (p=0.000 n=10+10)
    select           32.0µs ± 0%  33.1µs ± 1%   +3.25%  (p=0.000 n=10+10)
    def              58.0ns ± 0%  55.0ns ± 0%   -5.17%  (p=0.000 n=10+10)
    func_def         43.9µs ± 1%  44.4µs ± 2%   +1.20%  (p=0.007 n=10+10)
    call             63.5ns ± 1%  64.0ns ± 2%     ~     (p=0.307 n=10+10)
    func_call        1.08µs ± 1%  1.06µs ± 0%   -2.55%  (p=0.000 n=10+8)
    try_finally       142ns ± 0%   136ns ± 0%   -4.23%  (p=0.000 n=10+9)
    defer            2.32µs ± 1%  2.29µs ± 1%   -0.96%  (p=0.000 n=10+10)
    workgroup_empty  90.3µs ± 0%  73.8µs ± 1%  -18.29%  (p=0.000 n=10+10)
    workgroup_raise   108µs ± 1%    94µs ± 0%  -13.29%  (p=0.000 n=10+10)

(small changes are probably within noise; "go" and "workgroup_*" should be
representative)

The next patch will add `go` to Pyx API and correspondingly C, C++ and Pyx
level tests for it that will go into _golang_test.pyx extension. For
C-level test we'll use C source file - to verify that libgolang.h could
be used by C projects with C compiler, while for C++ and Pyx-level tests
the sources will be in C++. Thus _golang_test.so will be build from both
C and C++ sources.

This creates a problem: distutils / setuptools use the _same_ compiler
to compile both C and C++ sources and only use C++ compiler at link
stage. Thus, as it is not possible to tune compiler that is used only
for C++ sources, and also as it is not possible to provide per-source
flags, when compiling C-level test, the compiler will be invoked with
`-std=c++11` option that we inject. Gcc tolerates that and only prints a
warning, but Clang considers that an error and gives:

	error: invalid argument '-std=c++11' not allowed with 'C'`

A proper fix would be to change the build system from distutils to
something more flexible that uses C++ compiler for C++ sources and C
compiler for C sources and allows to tune per-unit flags. However it is
not a small step at this stage.

-> Use workaround and tweak options that are used when compiling sources
depending on whether it is C or C++.

- Add sleep functionality to libgolang runtime;
- Implement sleep for thread and gevent runtimes. Thread runtime
  implements sleep independently of GIL, but only for POSIX for now;
- Switch golang.time py module into using golang.time pyx module.

As we are adding sleep, related functionality to query system about
"what is current time?" is also added.

Libgolang will be generic library and it will adapt itself to particular
execution environment by way of runtime plugin. This commit introduces
stubs for two such runtimes - "thread" and "gevent" - and initializes
libgolang with particular runtime depending on whether golang is
imported with gevent preactivated or not.

The runtimes themselves are empty at this step. We'll be adding runtime
functionality in the following patches.

Start transforming Pygolang from being pure Python project into project
that uses both Python and Cython and provides both Python and Cython API
to its users.

For Cython API there is a goal for it to be usable independently of
Python GIL - i.e. independently of Python runtime and limitations
imposed by Python's global interpreter lock. The motivation for this
work is wendelin.core v2, which, due to its design, would deadlock if it
tries to take the GIL in its pinner thread.

This commit brings bare minimum establishing libraries and build system.
It:

- Introduces C/C++ library libgolang. Libgolang will be serving most of
  nogil functionality and Pyx API will be a small wrapper over it. At
  this step Libgolang comes only with C-level panic;

- Introduces Pyx package golang that is aliased to Pyx package
  golang._golang . Users can do `from golang cimport ...` similarly to how
  they do py-level `from golang import ...`. At this step golang.pyx
  only wraps panic and provides way to transform C-level panic into Python
  exception;

- Introduces Py package golang.pyx.build . This package, similarly to
  golang.org/pkg/go/build, serves as the system to build Pyx packages -
  both external packages that use Pygolang in pyx mode, and to build
  Pygolang itself. The build, internally, is served by setuptools_dso
  and cython. An example how to use golang.pyx.build from external
  project comes in golang/pyx/testprog/golang_pyx_user/ and is used by
  tests to verify golang.pyx.build functionality;

- Introduces Pygolang build dependency to cython and setuptools_dso.

We can include *.py from top-level directories, not only under
X/testdata/, X/testprog/ ...

This test is veriyfing only how leaked goroutines behave, not go in
general.

test_go was doing the same what pyrun is doing + adjusting PYTHONPATH a
bit. Merge PYTHONPATH adjustment into pyrun and use pyrun uniformly.

pyrun runs command without piping output.
pyout runs command and returns its output.

We'll need to use this utility in other places; golang_test is the
central place for testing utilities.

gpython/gpython_test.py:23: 'subprocess' imported but unused
gpython/gpython_test.py:24: 'six.PY3' imported but unused

... and everything that whole pygolang depends on in pygolang[all].

The reason we do this: currently golang.perf.benchlib needs NumPy, but
it would be not ok to depend whole pygolang on numpy. Similarly pybench
needs pytest, but it would be not ok to require pytest unconditionally.

Thus allow users to specify what they need from pygolang via extras,
like pygolang[x.perf.benchlib] or pygolang[cmd.pybench], with
pygolang[all] serving as merge point for all subpackages dependencies.

TODO in the future we'll, hopefully, also provide accumulation points,
like pygolang[x.perf] which contains everything pygolang[x.perf.*]
depends on.

Note: core functionality and gpython are provided always unconditionally.

This test passes, but the functionality was not tested before. In
particular all tests were passing even with the following hand-edits:

    --- a/golang/__init__.py
    +++ b/golang/__init__.py
    @@ -627,7 +627,7 @@ def selected():
             sel = g.which
             if isinstance(sel, _SendWaiting):
                 if not sel.ok:
    -                panic("send on closed channel")
    +                panic("send on closed channel ZZZ")
                 return sel.sel_n, None

             if isinstance(sel, _RecvWaiting):

with added tests the bug is caught and reported:

    def test_select():
        N = 1000 # times to do repeated select/chan or select/select interactions

        # sync: close vs select(send)
        ch = chan()
        def _():
            waitBlocked(ch.send)
            ch.close()
        go(_)
>       with panics("send on closed channel"): select((ch.send, 0))

golang_test.py:353:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

self = <golang.golang_test.panics instance at 0x7fc1da66e5f0>, exc_type = <class 'golang._PanicError'>, exc_val = _PanicError('send on closed channel ZZZ',)
exc_tb = <traceback object at 0x7fc1dabc33b0>

    def __exit__(self, exc_type, exc_val, exc_tb):
        ok = self.raises.__exit__(exc_type, exc_val, exc_tb)
        if not ok:
            return ok
        # _PanicError raised - let's check panic argument
>       assert self.exc_info.value.args == (self.arg,)
E       AssertionError: assert ('send on clo...channel ZZZ',) == ('send on closed channel',)
E         At index 0 diff: 'send on closed channel ZZZ' != 'send on closed channel'
E         Use -v to get the full diff

golang_test.py:1032: AssertionError

For buffered channel _tryrecv, on success, was unlocking ch._mu too
early - before accessing ch._dataq with ch._dataq.append().

Without the fix, newly added test breaks as e.g.

    golang/golang_test.py::test_chan_buf_recv_vs_tryrecv_race Exception in thread Thread-3:
    Traceback (most recent call last):
      File "/usr/lib/python2.7/threading.py", line 801, in __bootstrap_inner
        self.run()
      File "/usr/lib/python2.7/threading.py", line 754, in run
        self.__target(*self.__args, **self.__kwargs)
      File "/home/kirr/src/tools/go/pygolang-master/golang/golang_test.py", line 317, in _
        assert (_, _rx) == (1, None), ('i%d' % i)
    AssertionError: i30
    assert (0, None) == (1, None)
      At index 0 diff: 0 != 1
      Full diff:
      - (0, None)
      ?  ^
      + (1, None)
      ?  ^

For buffered channel _trysend, on success, was unlocking ch._mu too
early - before accessing ch._dataq with ch._dataq.popleft().

Without the fix, newly added test breaks as e.g.

    golang/golang_test.py::test_chan_buf_send_vs_tryrecv_race Exception in thread Thread-3:
    Traceback (most recent call last):
      File "/usr/lib/python2.7/threading.py", line 801, in __bootstrap_inner
        self.run()
      File "/usr/lib/python2.7/threading.py", line 754, in run
        self.__target(*self.__args, **self.__kwargs)
      File "/home/kirr/src/tools/go/pygolang-master/golang/golang_test.py", line 256, in _
        assert (_, _rx) == (1, None)
    AssertionError: assert (0, 209) == (1, None)
      At index 0 diff: 0 != 1
      Full diff:
      - (0, 209)
      + (1, None)

    Exception in thread Thread-2:
    Traceback (most recent call last):
      File "/usr/lib/python2.7/threading.py", line 801, in __bootstrap_inner
        self.run()
      File "/usr/lib/python2.7/threading.py", line 754, in run
        self.__target(*self.__args, **self.__kwargs)
      File "/home/kirr/src/tools/go/pygolang-master/golang/golang_test.py", line 243, in _
        ch.send(i)
      File "/home/kirr/src/tools/go/pygolang-master/golang/__init__.py", line 340, in send
        ok = self._trysend(obj)
      File "/home/kirr/src/tools/go/pygolang-master/golang/__init__.py", line 417, in _trysend
        rx = self._dataq.popleft()
    IndexError: pop from an empty deque

Currently it behaves correctly, but this aspect will need to be
explicitly cared about when channel implementation moves to C. Add test
to make sure it won't regress.

Starting from b51b8d5d (select: Run tests more thoroughly) we are
running select subtests in repeated mode with N=1000. However not all
select subtests were run in repeated mode - for example e.g.
"non-blocking try send: not ok" was being run only once.

-> Rework all select subtests to be run in repeated mode to increase the
probability of catching bugs.

Channels implementation will soon be moved to C and will become
independent from Python runtime. This way while pygolang will still be
providing panic, hooking in and raising arbitrary Python-level exception
will become problematic. -> Rework "blocks forever" tests to rely on
just panic to prepare for that.

Chan implementation is going to be moved into C, and this way direct
access to chan internals won't be possible. C implementation will export
dedicated functions for chan tests which will be used.

Prepare for that and factor out retrieving len of chan recv/send queues
into separate functions, which will be adapted at the time of C port.

Nil channel will soon be changed to be represented by underlying NULL C
pointer and there won't be channel object for nil and correspondingly
recv/send queues won't be there. -> Prepare for that and don't check for
recvq/sendq on nil channel.

We provide gpython since 32a21d5b (gpython: Python interpreter with
support for lightweight threads), and golang module, since the
beginning, automatically uses gevent if it was installed via monkey
patching.

This currently pass trivially (since time.now = stdtime.now), but will
be useful once we get time.now implementation into our hands and move it into Cython.

After 9c260fde (time: New package that mirrors Go's time) we have
golang.time.now and golang.time.sleep and it makes it a more
self-dependent system if timing facility is used through golang.time
instead of outside std time module.

For now this is only a "cleanness" change, but will become important once
we start adding pyx-level nogil API to pygolang - there it will be
important to use golang.time.* for correctness.

Starting from e6bea2cf (sync: New package that mirrors Go's sync) there
is sync.WaitGroup and it was using threading.Event to signal that
waitgroup's counter dropped to zero. threading.Event is implemented in
Python's stdlib via threading.Condition, which in turn uses
threading.Lock and list of waiters to implement its functionality.
Which in turn is very similar to what golang.chan does internally with
semaphore and e.g. recv queue.

I noticed this while debugging sync test deadlock (see previous patch)
and suspecting bug in threading.Event for a moment. While it turned
there is no bug in threading.Event, it is better to rely on the common
functionality for similar tasks, and pygolang's channel perfectly
matches here the need to signal an event. By using our own code instead
of stdlib's threading we are likely becoming less bug-prone.

This also brings a small speedup:

    (on i5@1.80GHz)
    name             old time/op  new time/op  delta
    workgroup_empty   239µs ± 1%   220µs ± 1%  -7.62%  (p=0.000 n=10+9)
    workgroup_raise   275µs ± 2%   264µs ± 2%  -3.85%  (p=0.000 n=10+10)

sync.WorkGroup test was doing ctx.done() wait from under test mutex,
something like

    def _(ctx, i):
        with mu:
            ...
            if i == 0:
                raise RuntimeError()    # to cause ctx cancel
            ctx.done().recv()           # i=1 -> wait till ctx is canceled

but it can be a deadlock if T(i=1) runs first and enters
ctx.done().recv() before T(i=0) is run - then T(i=0) will block forever
waiting to lock mu.

This failure was not seen so far, probably because the time to go a new
thread/goroutine is relatively high. However one of upcoming patches,
where go is made faster, revealed this problem and, without the fix,
sync.WorkGroup test was regularly deadlocking.

The problem was there from sync.WorkGroup beginning - from
9ee7ba91 (sync += WorkGroup).

Fix the deadlock by waiting for ctx.done() outside of mu.