Make into_inner more useful for http2 transport

[rklaehn]

Currently the http2 transport has an into_inner only on the SendSink, because the RecvStream never sees the raw bytes. The framing and deserialization gets done in the forwarder.

The recv side should be changed so that the stream of raw bytes::Bytes is also available for into_inner.

Also, there should be utilities to turn a stream of bytes into an AsyncRead and a sink of bytes into an AsyncWrite. The former exists for futures, but unfortunately not for tokio.