TrackQueues: Convenience methods and extension (#7)

* Adds a uuid field to tracks and handles to make it easier to identify and match event sources after the fact.
* Adds optional feature "builtin-queue" to expose a queue on every driver, as a convenience for users who can guarantee they'll need a queue for every driver/call.
* Adds methods to queues to allow access to the currently running track handle, remove a specified queue entry, as well as to mutate the underlying queue from a closure.

examples/serenity/voice_events_queue/Cargo.toml

@@ -10,6 +10,7 @@ tracing-subscriber = "0.2"
 tracing-futures = "0.2"
 
 [dependencies.songbird]
+features = ["builtin-queue"]
 path = "../../../"
 
 [dependencies.serenity]