bpo-32296: Implement asyncio.get_event_loop and _get_running_loop in C. (#4827)

bpo-32296: Implement asyncio.get_event_loop and _get_running_loop in C. (#4827)

asyncio.get_event_loop(), and, subsequently asyncio._get_running_loop()
are one of the most frequently executed functions in asyncio.  They also
can't be sped up by third-party event loops like uvloop.

When implemented in C they become 4x faster.