Implement Runtime::block_on using oneshot

[jonhoo]

A new take on #328 in which the provided future is spawned on the runtime, and a oneshot channel is used to communicate the result back.

[carllerche]

This is a fine initial impl. However, what do you think about removing the free fn initially and only provide the Runtime::block_on in this PR? In the past, fns like block_on have been the source of confusion. So, keeping it a bit hidden might be good until the confusion can be explored further.

[jonhoo]

Sure, that sounds like a good idea. Do you want to just not export it, or remove the fn entirely?

[carllerche]

I would remove it entirely for now and leave only Runtime::block_on.

[carllerche]

Oh, I'd also ❤️ some tests 💃

Copying some of these would probably work: https://github.com/tokio-rs/tokio/blob/master/tests/current_thread.rs

[jonhoo]

@carllerche Done and done!

[carllerche]

Thanks 👍