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/mixer.rs

@@ -0,0 +1,32 @@
+use super::{Interconnect, UdpRxMessage, UdpTxMessage, WsMessage};
+
+use crate::{tracks::Track, Bitrate};
+use flume::Sender;
+use xsalsa20poly1305::XSalsa20Poly1305 as Cipher;
+
+pub(crate) struct MixerConnection {
+    pub cipher: Cipher,
+    pub udp_rx: Sender<UdpRxMessage>,
+    pub udp_tx: Sender<UdpTxMessage>,
+}
+
+impl Drop for MixerConnection {
+    fn drop(&mut self) {
+        let _ = self.udp_rx.send(UdpRxMessage::Poison);
+        let _ = self.udp_tx.send(UdpTxMessage::Poison);
+    }
+}
+
+pub(crate) enum MixerMessage {
+    AddTrack(Track),
+    SetTrack(Option<Track>),
+    SetBitrate(Bitrate),
+    SetMute(bool),
+    SetConn(MixerConnection, u32),
+    DropConn,
+    ReplaceInterconnect(Interconnect),
+    RebuildEncoder,
+
+    Ws(Option<Sender<WsMessage>>),
+    Poison,
+}