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.

build.rs

@@ -0,0 +1,23 @@
+#[cfg(all(feature = "driver", not(any(feature = "rustls", feature = "native"))))]
+compile_error!(
+    "You have the `driver` feature enabled: \
+    either the `rustls` or `native` feature must be
+    selected to let Songbird's driver use websockets.\n\
+    - `rustls` uses Rustls, a pure Rust TLS-implemenation.\n\
+    - `native` uses SChannel on Windows, Secure Transport on macOS, \
+    and OpenSSL on other platforms.\n\
+    If you are unsure, go with `rustls`."
+);
+
+#[cfg(all(
+    feature = "twilight",
+    not(any(feature = "simd-zlib", feature = "stock-zlib"))
+))]
+compile_error!(
+    "Twilight requires you to specify a zlib backend: \
+    either the `simd-zlib` or `stock-zlib` feature must be
+    selected.\n\
+    If you are unsure, go with `stock-zlib`."
+);
+
+fn main() {}