Support AnyIO

[davidbrochart]

Thanks for this great library! In #1063 I noticed that more async functionalities have been added, and I was wondering if now would not be a good time to switch to AnyIO instead of asyncio, rather than later when it will be more difficult if async starts to spread out into the code base. Currently it seems to be just create_task(), gather() and run() that are used, so this should be easy to convert to AnyIO. AnyIO implements structured concurrency on top of asyncio, and gives Trio support for free. What are your thoughts about that?