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/command.rs

@@ -0,0 +1,53 @@
+use super::*;
+use crate::events::EventData;
+use std::time::Duration;
+use tokio::sync::oneshot::Sender as OneshotSender;
+
+/// A request from external code using a [`TrackHandle`] to modify
+/// or act upon an [`Track`] object.
+///
+/// [`Track`]: struct.Track.html
+/// [`TrackHandle`]: struct.TrackHandle.html
+pub enum TrackCommand {
+    /// Set the track's play_mode to play/resume.
+    Play,
+    /// Set the track's play_mode to pause.
+    Pause,
+    /// Stop the target track. This cannot be undone.
+    Stop,
+    /// Set the track's volume.
+    Volume(f32),
+    /// Seek to the given duration.
+    ///
+    /// On unsupported input types, this can be fatal.
+    Seek(Duration),
+    /// Register an event on this track.
+    AddEvent(EventData),
+    /// Run some closure on this track, with direct access to the core object.
+    Do(Box<dyn FnOnce(&mut Track) + Send + Sync + 'static>),
+    /// Request a read-only view of this track's state.
+    Request(OneshotSender<Box<TrackState>>),
+    /// Change the loop count/strategy of this track.
+    Loop(LoopState),
+}
+
+impl std::fmt::Debug for TrackCommand {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> Result<(), std::fmt::Error> {
+        use TrackCommand::*;
+        write!(
+            f,
+            "TrackCommand::{}",
+            match self {
+                Play => "Play".to_string(),
+                Pause => "Pause".to_string(),
+                Stop => "Stop".to_string(),
+                Volume(vol) => format!("Volume({})", vol),
+                Seek(d) => format!("Seek({:?})", d),
+                AddEvent(evt) => format!("AddEvent({:?})", evt),
+                Do(_f) => "Do([function])".to_string(),
+                Request(tx) => format!("Request({:?})", tx),
+                Loop(loops) => format!("Loop({:?})", loops),
+            }
+        )
+    }
+}