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/driver/tasks/message/mod.rs

@@ -0,0 +1,49 @@
+mod core;
+mod events;
+mod mixer;
+mod udp_rx;
+mod udp_tx;
+mod ws;
+
+pub(crate) use self::{core::*, events::*, mixer::*, udp_rx::*, udp_tx::*, ws::*};
+
+use flume::Sender;
+use tracing::info;
+
+#[derive(Clone, Debug)]
+pub(crate) struct Interconnect {
+    pub core: Sender<CoreMessage>,
+    pub events: Sender<EventMessage>,
+    pub mixer: Sender<MixerMessage>,
+}
+
+impl Interconnect {
+    pub fn poison(&self) {
+        let _ = self.events.send(EventMessage::Poison);
+    }
+
+    pub fn poison_all(&self) {
+        self.poison();
+        let _ = self.mixer.send(MixerMessage::Poison);
+    }
+
+    pub fn restart_volatile_internals(&mut self) {
+        self.poison();
+
+        let (evt_tx, evt_rx) = flume::unbounded();
+
+        self.events = evt_tx;
+
+        let ic = self.clone();
+        tokio::spawn(async move {
+            info!("Event processor restarted.");
+            super::events::runner(ic, evt_rx).await;
+            info!("Event processor finished.");
+        });
+
+        // Make mixer aware of new targets...
+        let _ = self
+            .mixer
+            .send(MixerMessage::ReplaceInterconnect(self.clone()));
+    }
+}