Implement new substream type for WebRTC #41
Description
WebRTC uses a channel-backed substream since support for WebRTC is implemented using str0m, which is a sans I/O library. This new substream type needs to implement tokio::AsyncWrite + tokio::AsyncRead and be integrated into the existing substream code that lives in src/substream/mod.rs. The new substream must support backpressure and the maximum message size is defined in the spec. See also the section about head-of-line blocking