Voice `CloseCode`s now map to a type rather than a collection of constants. Correct close code handling in this way terminates the websocket task when there is no likelihood of resuming, which was causing leftover tasks to spin at the `tokio::select` in some circumstances (i.e., ::leave, which keeps the `Driver` alive).
This implements a proof-of-concept for an improved audio frontend. The largest change is the introduction of events and event handling: both by time elapsed and by track events, such as ending or looping. Following on from this, the library now includes a basic, event-driven track queue system (which people seem to ask for unusually often). A new sample, `examples/13_voice_events`, demonstrates both the `TrackQueue` system and some basic events via the `~queue` and `~play_fade` commands.
Locks are removed from around the control of `Audio` objects, which should allow the backend to be moved to a more granular futures-based backend solution in a cleaner way.
