Async review

[SethDusek]

Hi, since I need this crate for a project I'm working on, I decided to give implementing futures Stream for a BusReceiver (Bus sending isn't async in this patch), however I am curious as to whether this implementation is correct, since I'm not too experienced with lock-free code

lib.rs

Basically, if a value is not ready, it sends the current Waker (kind of equivalent of thread handle) to the unparking channel, which will wake it up when it's ready.

[jonhoo]

I think @habnabit would be a good person to ping here, as they did a bunch of related work over in #15, culminating in https://github.com/habnabit/bus/tree/async_bus