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/tracks/mode.rs

@@ -0,0 +1,37 @@
+#[derive(Clone, Copy, Debug, Eq, PartialEq)]
+/// Playback status of a track.
+pub enum PlayMode {
+    /// The track is currently playing.
+    Play,
+    /// The track is currently paused, and may be resumed.
+    Pause,
+    /// The track has been manually stopped, and cannot be restarted.
+    Stop,
+    /// The track has naturally ended, and cannot be restarted.
+    End,
+}
+
+impl PlayMode {
+    /// Returns whether the track has irreversibly stopped.
+    pub fn is_done(self) -> bool {
+        matches!(self, PlayMode::Stop | PlayMode::End)
+    }
+
+    pub(crate) fn change_to(self, other: Self) -> PlayMode {
+        use PlayMode::*;
+
+        // Idea: a finished track cannot be restarted -- this action is final.
+        // We may want to change this in future so that seekable tracks can uncancel
+        // themselves, perhaps, but this requires a bit more machinery to readd...
+        match self {
+            Play | Pause => other,
+            state => state,
+        }
+    }
+}
+
+impl Default for PlayMode {
+    fn default() -> Self {
+        PlayMode::Play
+    }
+}