Allow re-entry into the event loop using tokens

[agronholm]

Trio allows using trio.from_thread.run and trio.from_thread.run_sync to be called from unrelated threads as long as a token (from trio.low_level.current.token) is provided.

On asyncio this token could be the event loop object itself, or another object that wraps it.

This change would likely allow us to remove the __new__() hack from BlockingPortal.

[arthur-tacca]

For applications where all that is needed is fire and forget, the implementations could be trivial, even assuming today's current_token() implementation (returns the TrioToken in Trio, and the event loop in asyncio).

In trio:

def run_sync_soon_from_thread(token, sync_fn, *args):
    assert isinstance(token, trio.lowlevel.TrioToken)
    token.run_sync_soon(sync_fn, *args)

In asyncio:

def run_sync_soon_from_thread(token, sync_fn, *args):
    assert isinstance(token, asyncio.AbstractEventLoop)
    token.call_soon_threadsafe(sync_fn, *args)

The only slight niggle is that if the loop has exited then asyncio raises RuntimeError whereas Trio raises trio.RunFinishedError, but the latter derives from RuntimeError so that could be the documented exception type.

More complex applications need to wait until the sync_fn has finished or allow async callbacks, and I recognise that's trickier.