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.

Cargo.toml

@@ -0,0 +1,155 @@
+[package]
+authors = ["Kyle Simpson <kyleandrew.simpson@gmail.com>"]
+description = "An async Rust library for the Discord voice API."
+documentation = "https://docs.rs/songbird"
+edition = "2018"
+homepage = "https://github.com/serenity-rs/serenity"
+include = ["src/**/*.rs", "Cargo.toml"]
+keywords = ["discord", "api", "rtp", "audio"]
+license = "ISC"
+name = "songbird"
+readme = "README.md"
+repository = "https://github.com/serenity-rs/serenity.git"
+version = "0.1.0"
+
+[dependencies]
+serde = { version = "1", features = ["derive"] }
+serde_json = "1"
+tracing = "0.1"
+tracing-futures = "0.2"
+
+[dependencies.async-trait]
+optional = true
+version = "0.1"
+
+[dependencies.async-tungstenite]
+default-features = false
+features = ["tokio-runtime"]
+optional = true
+version = "0.9"
+
+[dependencies.audiopus]
+optional = true
+version = "0.2"
+
+[dependencies.byteorder]
+optional = true
+version = "1"
+
+[dependencies.discortp]
+features = ["discord-full"]
+optional = true
+version = "0.2"
+
+[dependencies.flume]
+optional = true
+version = "0.9"
+
+[dependencies.futures]
+version = "0.3"
+
+[dependencies.parking_lot]
+optional = true
+version = "0.11"
+
+[dependencies.rand]
+optional = true
+version = "0.7"
+
+[dependencies.serenity]
+optional = true
+features = ["voice", "gateway"]
+path = "../"
+version = "0.9.0-rc.2"
+
+[dependencies.serenity-voice-model]
+optional = true
+path = "../voice-model"
+version = "0.9.0-rc.2"
+
+[dependencies.spin_sleep]
+optional = true
+version = "1"
+
+[dependencies.streamcatcher]
+optional = true
+version = "0.1"
+
+[dependencies.tokio]
+optional = true
+version = "0.2"
+default-features = false
+
+[dependencies.twilight-gateway]
+optional = true
+version = "0.1"
+default-features = false
+
+[dependencies.twilight-model]
+optional = true
+version = "0.1"
+default-features = false
+
+[dependencies.url]
+optional = true
+version = "2"
+
+[dependencies.xsalsa20poly1305]
+optional = true
+version = "0.5"
+
+[dev-dependencies]
+criterion = "0.3"
+utils = { path = "utils" }
+
+[features]
+default = [
+    "serenity-rustls",
+    "driver",
+    "gateway",
+]
+gateway = [
+    "flume",
+    "parking_lot",
+    "tokio/sync",
+]
+driver = [
+    "async-trait",
+    "async-tungstenite",
+    "audiopus",
+    "byteorder",
+    "discortp",
+    "flume",
+    "parking_lot",
+    "rand",
+    "serenity-voice-model",
+    "spin_sleep",
+    "streamcatcher",
+    "tokio/fs",
+    "tokio/io-util",
+    "tokio/net",
+    "tokio/rt-core",
+    "tokio/time",
+    "tokio/process",
+    "tokio/sync",
+    "url",
+    "xsalsa20poly1305",
+]
+rustls = ["async-tungstenite/tokio-rustls"]
+native = ["async-tungstenite/tokio-native-tls"]
+serenity-rustls = ["serenity/rustls_backend", "rustls", "gateway", "serenity-deps"]
+serenity-native = ["serenity/native_tls_backend", "native", "gateway", "serenity-deps"]
+twilight-rustls = ["twilight", "twilight-gateway/rustls", "rustls", "gateway"]
+twilight-native = ["twilight", "twilight-gateway/native", "native", "gateway"]
+twilight = ["twilight-model"]
+simd-zlib = ["twilight-gateway/simd-zlib"]
+stock-zlib = ["twilight-gateway/stock-zlib"]
+serenity-deps = ["async-trait"]
+
+[[bench]]
+name = "mixing"
+path = "benches/mixing.rs"
+harness = false
+
+[package.metadata.docs.rs]
+all-features = true