Driver: Split receive into its own feature (#141)

Adds the "receive" feature, which is disabled by default. When this is disabled, the UDP receive task is not compiled and not run, and as an optimisation the UDP receive buffer size is set to 0. All related events are also removed.

This also removes the UDP Tx task, and moves packet and keepalive sends back into the mixer thread. This allows us to entirely remove channels and various allocations between the mixer and an async task created only for sending data (i.e., fewer memcopies).

If "receive" is enabled, UDP sends are now non-blocking due to technical constraints -- failure to send is non-fatal, but *will* drop affected packets. Given that blocking on a UDP send indicates that the OS cannot clear send buffers fast enough, this should alleviate OS load.

Closes #131.

src/config.rs

@@ -1,6 +1,8 @@
+#[cfg(feature = "receive")]
+use crate::driver::DecodeMode;
 #[cfg(feature = "driver")]
 use crate::{
-    driver::{retry::Retry, CryptoMode, DecodeMode, MixMode},
+    driver::{retry::Retry, CryptoMode, MixMode},
     input::codecs::*,
 };
 
@@ -29,7 +31,8 @@ pub struct Config {
     ///
     /// [`CryptoMode::Normal`]: CryptoMode::Normal
     pub crypto_mode: CryptoMode,
-    #[cfg(feature = "driver")]
+
+    #[cfg(all(feature = "driver", feature = "receive"))]
     /// Configures whether decoding and decryption occur for all received packets.
     ///
     /// If voice receiving voice packets, generally you should choose [`DecodeMode::Decode`].
@@ -45,6 +48,7 @@ pub struct Config {
     /// [`DecodeMode::Pass`]: DecodeMode::Pass
     /// [user speaking events]: crate::events::CoreEvent::SpeakingUpdate
     pub decode_mode: DecodeMode,
+
     #[cfg(feature = "gateway")]
     /// Configures the amount of time to wait for Discord to reply with connection information
     /// if [`Call::join`]/[`join_gateway`] are used.
@@ -58,14 +62,16 @@ pub struct Config {
     /// [`Call::join`]: crate::Call::join
     /// [`join_gateway`]: crate::Call::join_gateway
     pub gateway_timeout: Option<Duration>,
+
     #[cfg(feature = "driver")]
-    /// Configures the maximum amount of time to wait for an attempted voice
-    /// connection to Discord.
+    /// Configures whether the driver will mix and output stereo or mono Opus data
+    /// over a voice channel.
     ///
     /// Defaults to [`Stereo`].
     ///
     /// [`Stereo`]: MixMode::Stereo
     pub mix_mode: MixMode,
+
     #[cfg(feature = "driver")]
     /// Number of concurrently active tracks to allocate memory for.
     ///
@@ -79,6 +85,7 @@ pub struct Config {
     /// Changes to this field in a running driver will only ever increase
     /// the capacity of the track store.
     pub preallocated_tracks: usize,
+
     #[cfg(feature = "driver")]
     /// Connection retry logic for the [`Driver`].
     ///
@@ -87,6 +94,7 @@ pub struct Config {
     ///
     /// [`Driver`]: crate::driver::Driver
     pub driver_retry: Retry,
+
     #[cfg(feature = "driver")]
     /// Configures whether or not each mixed audio packet is [soft-clipped] into the
     /// [-1, 1] audio range.
@@ -101,12 +109,14 @@ pub struct Config {
     ///
     /// [soft-clipped]: https://opus-codec.org/docs/opus_api-1.3.1/group__opus__decoder.html#gaff99598b352e8939dded08d96e125e0b
     pub use_softclip: bool,
+
     #[cfg(feature = "driver")]
     /// Configures the maximum amount of time to wait for an attempted voice
     /// connection to Discord.
     ///
     /// Defaults to 10 seconds. If set to `None`, connections will never time out.
     pub driver_timeout: Option<Duration>,
+
     #[cfg(feature = "driver")]
     #[derivative(Debug = "ignore")]
     /// Registry of the inner codecs supported by the driver, adding audiopus-based
@@ -116,6 +126,7 @@ pub struct Config {
     ///
     /// [`CODEC_REGISTRY`]: static@CODEC_REGISTRY
     pub codec_registry: &'static CodecRegistry,
+
     #[cfg(feature = "driver")]
     #[derivative(Debug = "ignore")]
     /// Registry of the muxers and container formats supported by the driver.
@@ -142,7 +153,7 @@ impl Default for Config {
         Self {
             #[cfg(feature = "driver")]
             crypto_mode: CryptoMode::Normal,
-            #[cfg(feature = "driver")]
+            #[cfg(all(feature = "driver", feature = "receive"))]
             decode_mode: DecodeMode::Decrypt,
             #[cfg(feature = "gateway")]
             gateway_timeout: Some(Duration::from_secs(10)),
@@ -179,6 +190,7 @@ impl Config {
         self
     }
 
+    #[cfg(feature = "receive")]
     /// Sets this `Config`'s received packet decryption/decoding behaviour.
     #[must_use]
     pub fn decode_mode(mut self, decode_mode: DecodeMode) -> Self {