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/events/untimed.rs

@@ -0,0 +1,28 @@
+use super::*;
+
+#[derive(Clone, Copy, Debug, Eq, Hash, PartialEq)]
+/// Track and voice core events.
+///
+/// Untimed events persist while the `action` in [`EventData`]
+/// returns `None`.
+///
+/// [`EventData`]: struct.EventData.html
+pub enum UntimedEvent {
+    /// Untimed events belonging to a track, such as state changes, end, or loops.
+    Track(TrackEvent),
+    /// Untimed events belonging to the global context, such as finished tracks,
+    /// client speaking updates, or RT(C)P voice and telemetry data.
+    Core(CoreEvent),
+}
+
+impl From<TrackEvent> for UntimedEvent {
+    fn from(evt: TrackEvent) -> Self {
+        UntimedEvent::Track(evt)
+    }
+}
+
+impl From<CoreEvent> for UntimedEvent {
+    fn from(evt: CoreEvent) -> Self {
+        UntimedEvent::Core(evt)
+    }
+}