``_abc`` module is added.  It is a speedup module with C implementations for
various functions and methods in ``abc``.  Creating an ABC subclass and calling
``isinstance`` or ``issubclass`` with an ABC subclass are up to 1.5x faster.
In addition, this makes Python start-up up to 10% faster.

Note that the new implementation hides internal registry and caches, previously
accessible via private attributes ``_abc_registry``, ``_abc_cache``, and
``_abc_negative_cache``.  There are three debugging helper methods that can be
used instead ``_dump_registry``, ``_abc_registry_clear``, and
``_abc_caches_clear``.
