Provide sync.RWMutex that can be useful for cases when there are
multiple simultaneous readers and more seldom writer(s).

This implements readers-writer mutex with preference for writers
similarly to Go version.

Only io.EOF and io.ErrUnexpectedEOF for now.
Moved here from wcfs from wendelin.core.

A Python error can have links to other errors by means of both .Unwrap()
and .__cause__ . These ways are both explicit and so should be treated
by e.g. errors.Is as present in error's error chain.

It is a bit unclear, at least initially, how to linearise and order
error chain traversal in divergence points - for exception objects where
both .Unwrap() and .__cause__ are !None. However more closer look
suggests linearisation rule to traverse into .__cause__ after going
through .Unwrap() part - please see details in documentation added into
_error.pyx

-> Teach errors.Is to do this traversal, and this way now e.g. exception
raised as

	raise X from Y

will be treated by errors.Is as being both X and Y, even if any of X or Y
also has its own error chain via .Unwrap().

Top-level documentation is TODO.

Following errors model in Go and fd95c88a (golang, errors, fmt: Error
chaining (C++/Pyx)) let's add support at Python-level for errors to wrap
each other and to be inspected/unwrapped:

- an error can additionally provide way to unwrap itself, if it
  provides .Unwrap() method. .__cause__ is not taken into account yet,
  but will be in a follow-up patch;
- errors.Is(err) tests whether an item in error's chain matches target;
- `fmt.Errorf("... : %w", ... err)` is similar to `"... : %s" % (..., err)`
  but resulting error, when unwrapped, will return err.
- errors.Unwrap is not exposed as chaining through both .Unwrap() and
  .__cause__ will need more than just "current element" as unwrapping
  state (i.e. errors.Unwrap API is insufficient - see next patch), and
  in practice users of errors.Unwrap() are very seldom.

Support for error chaining through .__cause__ will follow in the next
patch.

Top-level documentation is TODO.

See https://blog.golang.org/go1.13-errors for error chaining overview.

It is surprising to have an exception class that cannot be derived from.

Besides, in the future we'll use subclassing from golang.error as an
indicator that an error is a "well-defined" (in simple words - does not
need traceback to be interpreted).

The first step to expose errors and error chaining to Python:

- Add pyerror that wraps a pyx/nogil C-level error and is exposed as golang.error at py level.
- py errors must be compared by ==, not by "is"
- Add (py) errors.New to create a new error from text.
- a C-level error that has .Unwrap, is exposed with .Unwrap at py level,
  but full py-level chaining will be implemented in a follow-up patch.
- py error does not support inheritance yet.

Top-level documentation is TODO.

Following errors model in Go, let's add support for errors to wrap other
errors and to be inspected/unwrapped:

- an error can additionally provide way to unwrap itself, if it
  implements errorWrapper interface;
- errors.Unwrap(err) tries to extract wrapped error;
- errors.Is(err) tests whether an item in error's chain matches target;
- `fmt.errorf("... : %w", ... err)` is similar to `fmt.errorf("... : %s", ... err.c_str())`
  but resulting error, when unwrapped, will return err.

Add C++ implementation for the above + tests.
Python analogs will follow in the next patches.

Top-level documentation is TODO.

See https://blog.golang.org/go1.13-errors for error chaining overview.

Package cxx was added in 9785f2d3 (cxx: New package), but the interface
that cxx:dict provided turned out to be not optimal:

    dict.get  was returning (v, ok), and
    dict.pop  ----//---

Correct dict.get and dict.pop to return just value, and, similarly to
channels API, provide additional dict.get_ and dict.pop_ - extended
versions that also return ok:

    dict.get(k)  -> v
    dict.pop(k)  -> v
    dict.get_(k) -> (v, ok)
    dict.pop_(k) -> (v, ok)

This time add tests.

Follow the scheme established and used for all other packages, because
we will soon have fmt pyx part which, if named as fmt.pyx, will
intersect and conflict with fmt.py .

errors.New was added in a245ab56 (errors: New package) without test.

Makes understanding which test is it and where when one fails.

Currently libgolang_test.cpp contains tests for code in libgolang.cpp and
for code that lives in other libgolang packages - sync, fmt, etc. It is
becoming tight and we are going to split libgolang_test.cpp and move
package tests to their corresponing files - e.g. to sync_test.cpp and
the like.

Move common assertion utilities into shared header before that as a
preparatory step.

Just use builtins and cimported things that we have at pyx level.

U is preffered way to make sure an object is unicode string.

This will allow to integrate qq with u in the next patch.

Moving to compiled code for string processing functions is also
generally better for performance.

With Python3 I've got tired to constantly use .encode() and .decode();
getting exception if original argument was unicode on e.g. b.decode();
getting exception on raw bytes that are invalid UTF-8, not being able to
use bytes literal with non-ASCII characters, etc.

So instead of this pain provide two functions that make sure an object
is either bytes or unicode:

- b converts str/unicode/bytes s to UTF-8 encoded bytestring.

	Bytes input is preserved as-is:

	   b(bytes_input) == bytes_input

	Unicode input is UTF-8 encoded. The encoding always succeeds.
	b is reverse operation to u - the following invariant is always true:

	   b(u(bytes_input)) == bytes_input

- u converts str/unicode/bytes s to unicode string.

	Unicode input is preserved as-is:

	   u(unicode_input) == unicode_input

	Bytes input is UTF-8 decoded. The decoding always succeeds and input
	information is not lost: non-valid UTF-8 bytes are decoded into
	surrogate codes ranging from U+DC80 to U+DCFF.
	u is reverse operation to b - the following invariant is always true:

	   u(b(unicode_input)) == unicode_input

NOTE: encoding _and_ decoding *never* fail nor loose information. This
is achieved by using 'surrogateescape' error handler on Python3, and
providing manual fallback that behaves the same way on Python2.

The naming is chosen with the idea so that b(something) resembles
b"something", and u(something) resembles u"something".

This, even being only a part of strings solution discussed in [1],
should help handle byte- and unicode- strings in more robust and
distraction free way.

Top-level documentation is TODO.

[1] nexedi/zodbtools!13

This continues 60f6db6f (libgolang: Provide nil as alias for nullptr and
NULL): I've tried to compile pygolang with Clang on my Debian 10
workstation and got:

    $ CC=clang CXX=clang++ python setup.py build_dso -i

    In file included from ./golang/fmt.h:32:
    ./golang/libgolang.h:381:11: error: unknown type name 'nullptr_t'; did you mean 'std::nullptr_t'?
    constexpr nullptr_t nil = nullptr;
              ^~~~~~~~~
              std::nullptr_t
    /usr/bin/../lib/gcc/x86_64-linux-gnu/8/../../../../include/x86_64-linux-gnu/c++/8/bits/c++config.h:242:29: note: 'std::nullptr_t' declared here
      typedef decltype(nullptr)     nullptr_t;
                                    ^
    :
    In file included from ./golang/context.h
    In file included from golang/runtime/libgolang.cpp:30:
    ./golang/libgolang.h:381:11: error: unknown type name 'nullptr_t'; did you mean 'std::nullptr_t'?
    constexpr nullptr_t nil = nullptr;
              ^~~~~~~~~
              std::nullptr_t
    /usr/bin/../lib/gcc/x86_64-linux-gnu/8/../../../../include/x86_64-linux-gnu/c++/8/bits/c++config.h:242:29: note: 'std::nullptr_t' declared here
      typedef decltype(nullptr)     nullptr_t;
                                    ^
    :39:
    ./golang/libgolang.h:381:11: error: unknown type In file included from golang/fmt.cpp:25:
    In file included from ./golang/fmt.h:32:
    ./golang/libgolang.h:421:17: error: unknown type name 'nullptr_t'; did you mean 'std::nullptr_t'?
        inline chan(nullptr_t) { _ch = nil; }
                    ^~~~~~~~~
                    std::nullptr_t

    ...

It seems with GCC and Clang under macOS nullptr_t is automatically provided in
builtin namespace, while with older Clang on Linux (clang version 7.0.1-8) only
in std:: namespace - rightfully as nullptr_t is described to be present there:

https://en.cppreference.com/w/cpp/types/nullptr_t

This way we either have to correct all occurrences of nullptr_t to
std::nullptr_t, or do something similar with providing nil under golang:: .

To reduce noise I prefer the later and let it be named as Nil.

The code was assigning nil to local, _not_ global _tblockforever. As a
result _tblockforever was left set with a test hook even after leaving
test context. Fix it.

The bug was there starting from 3b241983 (Port/move channels to
C/C++/Pyx).

Had to change `= nil` to `= NULL` because with nil Cython complains as

        def __exit__(pypanicWhenBlocked t, typ, val, tb):
            global _tblockforever
            _tblockforever = nil
                            ^
    ------------------------------------------------------------

    golang/_golang_test.pyx:86:25: Cannot assign type 'nullptr_t' to 'void (*)(void) nogil'

This is https://github.com/cython/cython/issues/3314.

It's a leftover originating from b073f6df (time: Move/Port timers to
C++/Pyx nogil).

Instead of `pyctx.ctx = nil` it was just `ctx = nil` - i.e. assign nil
to local variable instead of changing pyctx instance data. We were not
observing this bug because Cython, for C++ fields of cdef classes,
automatically emits in-place destructor calls in generated __dealloc__

    https://github.com/cython/cython/blob/0.29.14-11-g8c620c388/Cython/Compiler/ModuleNode.py#L1477-L1478

and so this way there was no leak. However we want to be explicit and the
code was not correct. Fix it.

The bug was there from 2a359791 (context: Move/Port context package to
C++/Pyx nogil).

Convert Pyx part of the project to use nil instead of NULL.

Not every usage of NULL was converted and some places were left to use
NULL where changing it to nil currently hits Cython compilation error:

https://github.com/cython/cython/issues/3314

Convert C++ part of the project to use nil instead of NULL/nullptr.

We do not convert pyx part yet, because Cython currently does not
understand that nullptr_t has properties of NULL and with e.g. the
following change

    --- a/golang/_context.pyx
    +++ b/golang/_context.pyx
    @@ -116,7 +116,7 @@ cdef cppclass _PyValue (_interface, gobject) nogil:
         __dealloc__():
             with gil:
                 obj = <object>this.pyobj
    -            this.pyobj = NULL
    +            this.pyobj = nil
                 Py_DECREF(obj)

errors as

    Error compiling Cython file:
    ------------------------------------------------------------
    ...
            if __decref():
                del self
        __dealloc__():
            with gil:
                obj = <object>this.pyobj
                this.pyobj = nil
                            ^
    ------------------------------------------------------------

    golang/_context.pyx:119:25: Cannot assign type 'nullptr_t' to 'PyObject *'

https://github.com/cython/cython/issues/3314

Nil is more native to Go.

https://godoc.org/lab.nexedi.com/kirr/go123/xsync#WorkGroup
kirr/go123@515a6d14

Projects that use pyx.build (ex. wendelin.core) need recent
setuptools_dso fixes:

https://github.com/mdavidsaver/setuptools_dso/issues/5
https://github.com/mdavidsaver/setuptools_dso/commit/67d717a6
https://github.com/mdavidsaver/setuptools_dso/commit/e40f5883
https://github.com/mdavidsaver/setuptools_dso/commit/40b492ab

Increase setuptools_dso version in pygolang's build requirement for
uniformity as well.

Providing pygolang-specific DSO is needed because using just
setuptools_dso.DSO in external project will result in that e.g.
"<golang/libgolang.h>" won't be found.

This release is driven by wendelin.core v2 needs with one of the changes
being that now most of the library was moved into nogil code and can be
used fully from inside nogil world(*). Python modules are now just wrappers
of their nogil counterparts. The way for Python and nogil worlds to
communicate is also provided.

The move to nogil required many other enhancements along the way. Please
see CHANGELOG for overview.

The move to nogil brought some speedup automatically.
Below are benchmark results of this release compared to pygolang v0.0.4
(1573d101) for python-level benchmarks (we have only those at present):

	(on i7@2.6GHz)

thread runtime:

    name             old time/op  new time/op  delta
    go               18.3µs ± 0%  18.3µs ± 1%     ~     (p=1.000 n=10+10)
    chan             2.91µs ± 3%  2.99µs ± 5%   +2.73%  (p=0.022 n=10+10)
    select           3.57µs ± 3%  3.57µs ± 4%     ~     (p=0.720 n=9+10)
    def              55.0ns ± 0%  54.0ns ± 0%   -1.82%  (p=0.002 n=8+10)
    func_def         43.8µs ± 2%  44.1µs ± 1%   +0.64%  (p=0.035 n=10+9)
    call             64.0ns ± 0%  66.3ns ± 1%   +3.59%  (p=0.000 n=10+10)
    func_call        1.05µs ± 1%  1.24µs ± 0%  +17.80%  (p=0.000 n=10+7)
    try_finally       138ns ± 0%   137ns ± 1%   -0.51%  (p=0.003 n=10+10)
    defer            2.32µs ± 1%  2.63µs ± 1%  +13.52%  (p=0.000 n=10+10)
    workgroup_empty  38.0µs ± 1%  24.1µs ± 1%  -36.43%  (p=0.000 n=10+10)
    workgroup_raise  47.7µs ± 1%  28.2µs ± 0%  -40.76%  (p=0.000 n=10+10)

gevent runtime:

    name             old time/op  new time/op  delta
    go               16.9µs ± 1%  17.2µs ± 2%   +1.94%  (p=0.000 n=10+10)
    chan             7.43µs ± 0%  7.82µs ± 0%   +5.34%  (p=0.000 n=10+7)
    select           10.5µs ± 0%  11.2µs ± 0%   +6.74%  (p=0.000 n=10+10)
    def              63.0ns ± 0%  57.6ns ± 1%   -8.57%  (p=0.000 n=9+10)
    func_def         44.0µs ± 1%  44.2µs ± 1%     ~     (p=0.063 n=10+10)
    call             67.0ns ± 0%  64.0ns ± 0%   -4.48%  (p=0.002 n=8+10)
    func_call        1.06µs ± 1%  1.23µs ± 1%  +16.50%  (p=0.000 n=10+10)
    try_finally       144ns ± 0%   136ns ± 0%   -5.90%  (p=0.000 n=10+10)
    defer            2.37µs ± 1%  2.61µs ± 1%  +10.07%  (p=0.000 n=10+10)
    workgroup_empty  57.0µs ± 0%  55.0µs ± 2%   -3.53%  (p=0.000 n=10+9)
    workgroup_raise  72.4µs ± 0%  69.6µs ± 6%   -3.95%  (p=0.035 n=9+10)

workgroup_* changes for thread runtime is the speedup I am talking about.
defer/func_call slowdown is due to added exception chaining. We did not
optimize Python-level defer yet, and if/when that would be needed, it should
be possible to optimize by moving pydefer implementation into Cython.

(*) go and channels were moved into nogil world in Pygolang v0.0.3 +
v0.0.4 . Now it is the rest of the library that was moved with packages
like context, time, sync etc.

wendelin.core v2 needs nogil to run pinner thread on client side to
support isolation property in cooperation with wcfs: since there is a
`client -> wcfs -> pinner` loop:

      - - - - - -
     |           |
        pinner <------.
     |           |   wcfs
        client -------^
     |           |
      - - - - - -
     client process

the pinner thread would deadlock if it tries to take the GIL because
client thread can be holding GIL already while accessing wcfs-mmaped
memory (think doing e.g. `x = A[i]` in Python).

For base functionality we have overview in the readme itself, but for
packages we have only their listing with brief overview and no
documentation for in-package functionality.

Let's have at least links to .h/.pxd/.py where package functionality is
documented.

See commit 5a99b769 (libgolang: Start providing interfaces) for context.

Provide top-level documentation for memory management facilities that
was marked as TODO in refptr & co matches. See e.g. the following
commits for context:

- e82b4fab (libgolang: Objects refcounting (initial draft))
- b2253abf (libgolang: Rename refobj -> object)
- fd2a6fab (libgolang: Fix globals atexit race condition of ~refptr vs
  access from another thread)

defer is now part of libgolang API (see 1d153a45 "libgolang: Expose
defer as public C++ API"). It should be explained in top-level overview.

This amends commit b073f6df: clang was complaining that _Ticker/_Timer
were forward-declared as class, but later declared as struct:

    ./golang/time.h:107:1: warning: '_Ticker' defined as a struct here but previously declared as a class [-Wmismatched-tags]
    struct _Ticker : object {
    ^
    ./golang/time.h:77:1: note: did you mean struct here?
    class _Ticker;
    ^~~~~
    struct
    ./golang/time.h:140:1: warning: '_Timer' defined as a struct here but previously declared as a class [-Wmismatched-tags]
    struct _Timer : object {
    ^
    ./golang/time.h:78:1: note: did you mean struct here?
    class _Timer;
    ^~~~~
    struct

-> Fix it by using struct. Also used inline style inside refptr.

Race condition inside PyThread_release_lock should have been fixed on
both CPython/darwin and PyPy/darwin:

    https://github.com/python/cpython/commit/c5abd63e94fc
    https://bitbucket.org/pypy/pypy/commits/6cd7a0d1a940
    https://bitbucket.org/pypy/pypy/commits/a9d36d6af872

The bug is explained here:

    https://bugs.python.org/issue38106
    https://bitbucket.org/pypy/pypy/issues/3072

The following commits added tests for the bug on Pygolang side:

    34b7a1f4 (golang: Expose Sema and Mutex as public Python and Cython/nogil API)
    5142460d (libgolang/thread: Add links to upstream PyThread_release_lock bug)

Provide string utilities to verify whether string has suffix/prefix,
trim it, and split string by a delimiter. The code originated in wcfs
codebase in wendelin.core .

Pyx/nogil only.

With fmt.sprintf() and fmt.errorf() to format strings and errors. The
code was extracted from wcfs in wendelin.core .

Pyx/nogil only.

std::string is frequently too cumbersome. Providing string in golang::
namespace allows to have a more closer feel of a Go environment after
`using namespace golang`.

This ammends 33cf3113: Kill now unused imports.