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/id.rs

@@ -0,0 +1,121 @@
+//! Newtypes around Discord IDs for library cross-compatibility.
+
+#[cfg(feature = "driver")]
+use crate::model::id::{GuildId as DriverGuild, UserId as DriverUser};
+#[cfg(feature = "serenity")]
+use serenity::model::id::{
+    ChannelId as SerenityChannel,
+    GuildId as SerenityGuild,
+    UserId as SerenityUser,
+};
+use std::fmt::{Display, Formatter, Result as FmtResult};
+#[cfg(feature = "twilight")]
+use twilight_model::id::{
+    ChannelId as TwilightChannel,
+    GuildId as TwilightGuild,
+    UserId as TwilightUser,
+};
+
+/// ID of a Discord voice/text channel.
+#[derive(Clone, Copy, Debug, Default, Eq, Hash, PartialEq)]
+pub struct ChannelId(pub u64);
+
+/// ID of a Discord guild (colloquially, "server").
+#[derive(Clone, Copy, Debug, Default, Eq, Hash, PartialEq)]
+pub struct GuildId(pub u64);
+
+/// ID of a Discord user.
+#[derive(Clone, Copy, Debug, Default, Eq, Hash, PartialEq)]
+pub struct UserId(pub u64);
+
+impl Display for ChannelId {
+    fn fmt(&self, f: &mut Formatter<'_>) -> FmtResult {
+        Display::fmt(&self.0, f)
+    }
+}
+
+impl From<u64> for ChannelId {
+    fn from(id: u64) -> Self {
+        Self(id)
+    }
+}
+
+#[cfg(feature = "serenity")]
+impl From<SerenityChannel> for ChannelId {
+    fn from(id: SerenityChannel) -> Self {
+        Self(id.0)
+    }
+}
+
+#[cfg(feature = "twilight")]
+impl From<TwilightChannel> for ChannelId {
+    fn from(id: TwilightChannel) -> Self {
+        Self(id.0)
+    }
+}
+
+impl Display for GuildId {
+    fn fmt(&self, f: &mut Formatter<'_>) -> FmtResult {
+        Display::fmt(&self.0, f)
+    }
+}
+
+impl From<u64> for GuildId {
+    fn from(id: u64) -> Self {
+        Self(id)
+    }
+}
+
+#[cfg(feature = "serenity")]
+impl From<SerenityGuild> for GuildId {
+    fn from(id: SerenityGuild) -> Self {
+        Self(id.0)
+    }
+}
+
+#[cfg(feature = "driver")]
+impl From<GuildId> for DriverGuild {
+    fn from(id: GuildId) -> Self {
+        Self(id.0)
+    }
+}
+
+#[cfg(feature = "twilight")]
+impl From<TwilightGuild> for GuildId {
+    fn from(id: TwilightGuild) -> Self {
+        Self(id.0)
+    }
+}
+
+impl Display for UserId {
+    fn fmt(&self, f: &mut Formatter<'_>) -> FmtResult {
+        Display::fmt(&self.0, f)
+    }
+}
+
+impl From<u64> for UserId {
+    fn from(id: u64) -> Self {
+        Self(id)
+    }
+}
+
+#[cfg(feature = "serenity")]
+impl From<SerenityUser> for UserId {
+    fn from(id: SerenityUser) -> Self {
+        Self(id.0)
+    }
+}
+
+#[cfg(feature = "driver")]
+impl From<UserId> for DriverUser {
+    fn from(id: UserId) -> Self {
+        Self(id.0)
+    }
+}
+
+#[cfg(feature = "twilight")]
+impl From<TwilightUser> for UserId {
+    fn from(id: TwilightUser) -> Self {
+        Self(id.0)
+    }
+}