Go 1.16 has introduced net.ErrClosed, which should be returned/wrapped when an
I/O call is performed on a network connection which has already been closed.
This is useful to avoid cluttering logs with messages like "failed to close
WebSocket: already wrote close".

Closes: https://github.com/nhooyr/websocket/issues/286

Also ran gofmt on everything. Thanks again @paralin. #334

Co-authored-by: Christian Stewart <christian@paral.in>

This way we don't pollute the directory tree.

See https://github.com/nhooyr/websocket/issues/182

Closes #182

Closes #179

Too many improvements and changes to list.

Will include a detailed changelog for release.