Driver/receive: Implement audio reorder/jitter buffer (#156)

This PR Introduces a new `VoiceTick` event which collects and reorders all RTP packets to smooth over network instability, as well as to synchronise user audio streams. Raw packet events have been moved to `RtpPacket`, while `SpeakingUpdate`s have been removed as they can be easily computed using the `silent`/`speaking` audio maps included in each event.

Closes #146.

src/events/context/data/rtp.rs

@@ -0,0 +1,30 @@
+use discortp::rtp::RtpPacket;
+
+use super::*;
+
+#[derive(Clone, Debug, Eq, PartialEq)]
+#[non_exhaustive]
+/// Opus audio packet, received from another stream (detailed in `packet`).
+/// `payload_offset` contains the true payload location within the raw packet's `payload()`,
+/// if extensions or raw packet data are required.
+pub struct RtpData {
+    /// Raw RTP packet data.
+    ///
+    /// Includes the SSRC (i.e., sender) of this packet.
+    pub packet: Bytes,
+    /// Byte index into the packet body (after headers) for where the payload begins.
+    pub payload_offset: usize,
+    /// Number of bytes at the end of the packet to discard.
+    pub payload_end_pad: usize,
+}
+
+impl RtpData {
+    /// Create a zero-copy view of the inner RTP packet.
+    ///
+    /// This allows easy access to packet header fields, taking them from the underlying
+    /// `Bytes` as needed while handling endianness etc.
+    pub fn rtp(&'_ self) -> RtpPacket<'_> {
+        RtpPacket::new(&self.packet)
+            .expect("FATAL: leaked illegally small RTP packet from UDP Rx task.")
+    }
+}