Until now libgolang had semaphore and mutex functionality, but
implemented only internally and not exposed to users. Since for its
pinner thread wendelin.core v2 needs not only nogil channels, but also
nogil mutexes, timers and so on, it is now time to start providing that.

We start with mutexes:

- Expose Mutex from insides of libgolang.cpp to public API in
  libgolang.h . We actually expose both Mutex and Sema because
  semaphores are sometimes also useful for low-level synchronization,
  and because it is easier to export Mutex and Sema both at the same time.

- Provide pyx/nogil API to those as sync.Mutex and sync.Sema.

- Provide corresponding python wrappers.

- Add Pyx-level test for semaphore wakeup when wakeup is done not by the
  same thread which did the original acquire. This is the test that was
  promised in 5142460d (libgolang/thread: Add links to upstream
  PyThread_release_lock bug), and it used to corrupt memory and deadlock
  on macOS due to CPython & PyPy runtime bugs:

    https://bugs.python.org/issue38106
      -> https://github.com/python/cpython/pull/16047
    https://bitbucket.org/pypy/pypy/issues/3072

Note about python wrappers: At present, due to
https://github.com/cython/cython/issues/3165, C-level panic is not
properly translated into Py-level exception in (Py)Sema/Mutex constructors.
This should not pose a real problem in practice though.