Adapter to impl futures::Stream

[fibonacci1729]

This PR adds a futures::Stream adapter (gated on the futures-stream feature) allowing the conversion from RawStreamReader into RawStreamReaderStream which implements futures::Stream.

[alexcrichton]

Thanks! Mind putting this in a mod futures_stream; or similar to avoid the extra #[cfg]?

[alexcrichton]

Also, can you add some CI configuration and/or a test to ensure this builds?

[fibonacci1729]

@alexcrichton I moved the code into futures_stream.rs, added a build check in main.yml, and a test tests/runtime-async/async/stream-to-futures-stream.