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/constants.rs

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+//! Constants affecting driver function and API handling.
+
+#[cfg(feature = "driver")]
+use audiopus::{Bitrate, SampleRate};
+#[cfg(feature = "driver")]
+use discortp::rtp::RtpType;
+use std::time::Duration;
+
+#[cfg(feature = "driver")]
+/// The voice gateway version used by the library.
+pub const VOICE_GATEWAY_VERSION: u8 = crate::model::constants::GATEWAY_VERSION;
+
+#[cfg(feature = "driver")]
+/// Sample rate of audio to be sent to Discord.
+pub const SAMPLE_RATE: SampleRate = SampleRate::Hz48000;
+
+/// Sample rate of audio to be sent to Discord.
+pub const SAMPLE_RATE_RAW: usize = 48_000;
+
+/// Number of audio frames/packets to be sent per second.
+pub const AUDIO_FRAME_RATE: usize = 50;
+
+/// Length of time between any two audio frames.
+pub const TIMESTEP_LENGTH: Duration = Duration::from_millis(1000 / AUDIO_FRAME_RATE as u64);
+
+#[cfg(feature = "driver")]
+/// Default bitrate for audio.
+pub const DEFAULT_BITRATE: Bitrate = Bitrate::BitsPerSecond(128_000);
+
+/// Number of samples in one complete frame of audio per channel.
+///
+/// This is equally the number of stereo (joint) samples in an audio frame.
+pub const MONO_FRAME_SIZE: usize = SAMPLE_RATE_RAW / AUDIO_FRAME_RATE;
+
+/// Number of individual samples in one complete frame of stereo audio.
+pub const STEREO_FRAME_SIZE: usize = 2 * MONO_FRAME_SIZE;
+
+/// Number of bytes in one complete frame of raw `f32`-encoded mono audio.
+pub const MONO_FRAME_BYTE_SIZE: usize = MONO_FRAME_SIZE * std::mem::size_of::<f32>();
+
+/// Number of bytes in one complete frame of raw `f32`-encoded stereo audio.
+pub const STEREO_FRAME_BYTE_SIZE: usize = STEREO_FRAME_SIZE * std::mem::size_of::<f32>();
+
+/// Length (in milliseconds) of any audio frame.
+pub const FRAME_LEN_MS: usize = 1000 / AUDIO_FRAME_RATE;
+
+/// Maximum number of audio frames/packets to be sent per second to be buffered.
+pub const CHILD_BUFFER_LEN: usize = AUDIO_FRAME_RATE / 2;
+
+/// Maximum packet size for a voice packet.
+///
+/// Set a safe amount below the Ethernet MTU to avoid fragmentation/rejection.
+pub const VOICE_PACKET_MAX: usize = 1460;
+
+/// Delay between sends of UDP keepalive frames.
+///
+/// Passive monitoring of Discord itself shows that these fire every 5 seconds
+/// irrespective of outgoing UDP traffic.
+pub const UDP_KEEPALIVE_GAP_MS: u64 = 5_000;
+
+/// Type-converted delay between sends of UDP keepalive frames.
+///
+/// Passive monitoring of Discord itself shows that these fire every 5 seconds
+/// irrespective of outgoing UDP traffic.
+pub const UDP_KEEPALIVE_GAP: Duration = Duration::from_millis(UDP_KEEPALIVE_GAP_MS);
+
+/// Opus silent frame, used to signal speech start and end (and prevent audio glitching).
+pub const SILENT_FRAME: [u8; 3] = [0xf8, 0xff, 0xfe];
+
+/// The one (and only) RTP version.
+pub const RTP_VERSION: u8 = 2;
+
+#[cfg(feature = "driver")]
+/// Profile type used by Discord's Opus audio traffic.
+pub const RTP_PROFILE_TYPE: RtpType = RtpType::Dynamic(120);