Consider implementing `Stream` for Request / Response

[carllerche]

Consider implementing Stream for Request<T> / Response<T> where T: Stream.

This could be an optional dependency on the futures crate.

[seanmonstar]

Considering the instability of futures, this should probably not be considered for a couple years, so I'm going to close for now. That said, with request.into_body(), it seems trivial to get the Stream out.