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/crypto.rs

@@ -0,0 +1,38 @@
+//! Encryption schemes supported by Discord's secure RTP negotiation.
+
+/// Variants of the XSalsa20Poly1305 encryption scheme.
+///
+/// At present, only `Normal` is supported or selectable.
+#[derive(Clone, Copy, Debug, Eq, PartialEq)]
+#[non_exhaustive]
+pub enum Mode {
+    /// The RTP header is used as the source of nonce bytes for the packet.
+    ///
+    /// Equivalent to a nonce of at most 48b (6B) at no extra packet overhead:
+    /// the RTP sequence number and timestamp are the varying quantities.
+    Normal,
+    /// An additional random 24B suffix is used as the source of nonce bytes for the packet.
+    ///
+    /// Full nonce width of 24B (192b), at an extra 24B per packet (~1.2 kB/s).
+    Suffix,
+    /// An additional random 24B suffix is used as the source of nonce bytes for the packet.
+    ///
+    /// Nonce width of 4B (32b), at an extra 4B per packet (~0.2 kB/s).
+    Lite,
+}
+
+impl Mode {
+    /// Returns the name of a mode as it will appear during negotiation.
+    pub fn to_request_str(self) -> &'static str {
+        use Mode::*;
+        match self {
+            Normal => "xsalsa20_poly1305",
+            Suffix => "xsalsa20_poly1305_suffix",
+            Lite => "xsalsa20_poly1305_lite",
+        }
+    }
+}
+
+// TODO: implement encrypt + decrypt + nonce selection for each.
+// This will probably need some research into correct handling of
+// padding, reported length, SRTP profiles, and so on.