Voice Rework -- Events, Track Queues (#806)

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.

src/driver/tasks/message/core.rs

@@ -0,0 +1,24 @@
+use crate::{
+    driver::connection::error::Error,
+    events::EventData,
+    tracks::Track,
+    Bitrate,
+    ConnectionInfo,
+};
+use flume::Sender;
+
+#[allow(clippy::large_enum_variant)]
+#[derive(Debug)]
+pub enum CoreMessage {
+    ConnectWithResult(ConnectionInfo, Sender<Result<(), Error>>),
+    Disconnect,
+    SetTrack(Option<Track>),
+    AddTrack(Track),
+    SetBitrate(Bitrate),
+    AddEvent(EventData),
+    Mute(bool),
+    Reconnect,
+    FullReconnect,
+    RebuildInterconnect,
+    Poison,
+}