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/input/container/frame.rs

@@ -0,0 +1,8 @@
+/// Information used in audio frame detection.
+#[derive(Clone, Copy, Debug)]
+pub struct Frame {
+    /// Length of this frame's header, in bytes.
+    pub header_len: usize,
+    /// Payload length, in bytes.
+    pub frame_len: usize,
+}