Start new package that provides C++ amendments to be used by libgolang
and its users. Current functionality: dict and set that wrap
std::unordered_map and std::unordered_set into ergonomic interface.

The code originates from wendelin.core:
https://lab.nexedi.com/kirr/wendelin.core/blob/5a045ed1/wcfs/internal/wcfs_misc.h#L214-257

Pyx/nogil only.