Events: Break out and non-exhaust context body structs (#54)

This PR makes many of the types under `EventContext` separate `#[non_exhaustive]` structs. This makes it more feasible to add further information to connection and packet events as required in future. On this note, driver (re)connection events now include the SSRC supplied by Discord and the domain name which was connected to.

In addition, this fixes global timed events to return a list of all live tracks, and extensively details/documents events at a high level.

This was tested using `cargo make ready`.

examples/serenity/voice_receive/src/main.rs

@@ -84,35 +84,35 @@ impl VoiceEventHandler for Receiver {
                     speaking,
                 );
             },
-            Ctx::SpeakingUpdate {ssrc, speaking} => {
+            Ctx::SpeakingUpdate(data) => {
                 // You can implement logic here which reacts to a user starting
                 // or stopping speaking.
                 println!(
                     "Source {} has {} speaking.",
-                    ssrc,
-                    if *speaking {"started"} else {"stopped"},
+                    data.ssrc,
+                    if data.speaking {"started"} else {"stopped"},
                 );
             },
-            Ctx::VoicePacket {audio, packet, payload_offset, payload_end_pad} => {
+            Ctx::VoicePacket(data) => {
                 // An event which fires for every received audio packet,
                 // containing the decoded data.
-                if let Some(audio) = audio {
+                if let Some(audio) = data.audio {
                     println!("Audio packet's first 5 samples: {:?}", audio.get(..5.min(audio.len())));
                     println!(
                         "Audio packet sequence {:05} has {:04} bytes (decompressed from {}), SSRC {}",
-                        packet.sequence.0,
+                        data.packet.sequence.0,
                         audio.len() * std::mem::size_of::<i16>(),
-                        packet.payload.len(),
-                        packet.ssrc,
+                        data.packet.payload.len(),
+                        data.packet.ssrc,
                     );
                 } else {
                     println!("RTP packet, but no audio. Driver may not be configured to decode.");
                 }
             },
-            Ctx::RtcpPacket {packet, payload_offset, payload_end_pad} => {
+            Ctx::RtcpPacket(data) => {
                 // An event which fires for every received rtcp packet,
                 // containing the call statistics and reporting information.
-                println!("RTCP packet received: {:?}", packet);
+                println!("RTCP packet received: {:?}", data.packet);
             },
             Ctx::ClientConnect(
                 ClientConnect {audio_ssrc, video_ssrc, user_id, ..}