Add `Async#asyncPoll` for #3087

[seigert]

Implementation of proposed utility method in Async for double-check registration. Also implemented in IO itself and some tests.

I've chosen asyncPoll as a name as method itself reminds me about Rust's Future poll method with also returns computed result or provides waker to schedule another poll sometime later.

[seigert]

Also, should docs for cont be updated about this method? "If you are an implementor, and you have async (or asyncPool), Async.defaultCont provides an implementation of cont in terms of async/asyncPool."

[seigert]

@armanbilge, do you have any insight why check may randomly flick? Yesterday it was IoAppSpec, not it is WorkerThreadNameSpec -- both should be unaffected by my changes. :(

[armanbilge]

Darn, I just restarted CI for you. IOAppSpec is a well known flake. WorkerThreadNameSpec was only recently added, but I guess it's a flaky one too :(