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/state.rs

@@ -0,0 +1,31 @@
+use super::*;
+
+/// State of an [`Track`] object, designed to be passed to event handlers
+/// and retrieved remotely via [`TrackHandle::get_info`] or
+/// [`TrackHandle::get_info_blocking`].
+///
+/// [`Track`]: struct.Track.html
+/// [`TrackHandle::get_info`]: struct.TrackHandle.html#method.get_info
+/// [`TrackHandle::get_info_blocking`]: struct.TrackHandle.html#method.get_info_blocking
+#[derive(Copy, Clone, Debug, Default, PartialEq)]
+pub struct TrackState {
+    /// Play status (e.g., active, paused, stopped) of this track.
+    pub playing: PlayMode,
+    /// Current volume of this track.
+    pub volume: f32,
+    /// Current playback position in the source.
+    ///
+    /// This is altered by loops and seeks
+    pub position: Duration,
+    /// Total playback time, increasing monotonically.
+    pub play_time: Duration,
+    /// Remaining loops on this track.
+    pub loops: LoopState,
+}
+
+impl TrackState {
+    pub(crate) fn step_frame(&mut self) {
+        self.position += TIMESTEP_LENGTH;
+        self.play_time += TIMESTEP_LENGTH;
+    }
+}