[mypyc] Clear coroutine env on coroutine completion

[p-sawicki]

The env_class object associated with a mypyc coroutine is not immediately cleared when the coroutine completes. This can significantly increase memory usage, since the env class may hold references to captured locals and values spilled across suspension points.

The objects are eventually collectible by the GC but the collection might be delayed in cases where a nested coroutine is awaited, eg.

async def allocate(size: int) -> None:
    payload = bytearray(size)

    async def nested() -> int:
        return payload[-1]

    assert await nested() == 0

With nesting mypyc creates env classes for both allocate and nested that may reference each other and form a cycle.

To fix this, clear the env_class immediately after the coroutine completes. This matches behavior of cpython, which clears frames of completed coroutines immediately in the eval loop.