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/looping.rs

@@ -0,0 +1,22 @@
+#[derive(Copy, Clone, Debug, Eq, PartialEq)]
+/// Looping behaviour for a [`Track`].
+///
+/// [`Track`]: struct.Track.html
+pub enum LoopState {
+    /// Track will loop endlessly until loop state is changed or
+    /// manually stopped.
+    Infinite,
+
+    /// Track will loop `n` more times.
+    ///
+    /// `Finite(0)` is the `Default`, stopping the track once its [`Input`] ends.
+    ///
+    /// [`Input`]: ../input/struct.Input.html
+    Finite(usize),
+}
+
+impl Default for LoopState {
+    fn default() -> Self {
+        Self::Finite(0)
+    }
+}