Rewrite WebSockets code

[ranile]

The WebSockets API has been completely re-done. Instead of splitting the WebSocket into sender and receiver and using exposed mpsc channels, everything is self contained. The WebSocket struct now itself implements Sink and Stream.

Fixes: #2
Fixes: #3
Inspired by: rustwasm/gloo#113

TODOs (potentially in future PRs)

• Add a callback-styled API using builder pattern
• Implement AsyncRead/AsyncWrite
• Don't forget Closures - (WebSocket API leaks callbacks #2)