Reason through Future, Generator, Coroutine, Awaitable

[gvanrossum]

This came up in #1137 (comment). We should eventually know exactly how to declare the type of various coroutine-like things, whether they use @asyncio.coroutine, async def, or merely yield from.

[gvanrossum]

I think Future and Coroutine are owned by the asyncio stubs. The relationship and meaning of Generator and Iterator seems to have been sorted out. Support for yield from appears solid. We also have Awaitable, AsyncIterable and AsyncIterator in typing.py.

What remains IMO is support for async def and await (and async for, async with). For the return type of async def, see the discussion in the typing tracker.

[gvanrossum]

Closing in favor of #1453.