wai-websockets: dependency websockets 0.7.4.1 => 0.8.0.0

There's quite a big bump of `websockets` from `0.7.4.1` to `0.8.0.0`.

Legacy code structures are retained as much as possible though lots of things were rewritten, most notably the replacement of `enumerator` with `io-streams` used by `websockets`.