Add a single construct which both does async with and then async for

[evhub]

When working with async generators, it is often advised, using something like aclosing or trio_async_generator to implement a pattern like:

async with my_generator() as values:
    async for x in values:
        ...

which unnecessarily creates two nested blocks in a relatively non-ergonomic way. Instead, we could have a single syntax like

async with for x in my_generator():
    ...

that compiles to the code above.