Concat with async iterables

[tr11]

I'm trying to adapt the concat operator to implement the catch_exception op, but just noticed that the concat fails with an async iterable. I'm using the following observable:

async def asynciter():
    for i in range(5):
        await asyncio.sleep(1)
        yield i
xs = from_async_iterable(asynciter())

I have a fork with a working catch_exception and retry here, as well as the tests mentioned: tr11@6219f9a