As discussed in the #development channel in discord
- Switch from using message types to integer op codes
- Consolidate op-specific keys into `d` sub-object
- Shorten low-level payload keys from `messageType` to `op`, add `d`
Other changes:
- The WebSocketCloseCode enum has been refactored. It's best to just
treat it like it's new
- Some performance benefits came along the way. Nothing gamechanging,
but notable
- Various bug fixes discovered while refactoring