utils/asyn: Replace nest_asyncio with greenlet

Provide an implementation of re-entrant asyncio.run() that is less
brittle than what greenback provides (e.g. no use of ctypes to poke
extension types).

The general idea of the implementation consists in treating the executed
coroutine as a generator, then turning that generator into a generator
implemented using greenlet. This allows a nested function to make the
top-level parent yield values on its behalf, as if every call was
annotated with "yield from".

tests/test_asyn.py

@@ -17,6 +17,7 @@
 import sys
 import asyncio
 from functools import partial
+import contextvars
 from concurrent.futures import ThreadPoolExecutor
 from contextlib import contextmanager