Native split support without Sink

[joshtriplett]

Filing this based on the discussions starting at #152 (comment) . I don't plan to write this support myself, but if it existed I'd be thrilled to use it.

Currently, splitting a WebSocketStream into a write and read side requires Sink support and futures_util::stream::StreamExt::split.

To support splitting without that, we could have a native function WebSocketStream::split() returning a WebSocketSender that has the send function, and a WebSocketReceiver that implements Stream. If the sink feature is enabled, WebSocketSender can implement Sink.

With that support, ByteReader can continue to always expect a Stream. ByteWriter can support any Sink if the sink feature is enabled, or support only WebSocketStream and WebSocketSender otherwise (using a sealed trait).