Provide sync.WorkGroup that can be used directly from C++ and Pyx/nogil codes.
Python-level sync.WorkGroup becomes a wrapper around pyx/nogil one.

Like with context (2a359791 "context: Move/Port context package to
C++/Pyx nogil"), timers (b073f6df "time: Move/Port timers to C++/Pyx
nogil") and interfaces (5a99b769 "libgolang: Start providing
interfaces") memory for on-heap allocated WorkGroup is managed
automatically.

Python-level tests should be enough to cover C++/Pyx functionality at
zero-level approximation.