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.

examples/twilight/Cargo.toml

@@ -0,0 +1,21 @@
+[package]
+name = "basic-twilight-bot"
+version = "0.1.0"
+authors = ["Twilight and Serenity Contributors"]
+edition = "2018"
+
+[dependencies]
+futures = "0.3"
+tracing = "0.1"
+tracing-subscriber = "0.2"
+serde_json = { version = "1" }
+tokio = { features = ["macros", "rt-threaded", "sync"], version = "0.2" }
+twilight-gateway = "0.1"
+twilight-http = "0.1"
+twilight-model = "0.1"
+twilight-standby = "0.1"
+
+[dependencies.songbird]
+path = "../.."
+default-features = false
+features = ["twilight-rustls", "gateway", "driver", "stock-zlib"]