Changes a stored `Arc` pointer to the call (used to queue up further tracks in response to events) into a `Weak`, as the event handler's strong pointer would keep the Call and Driver objects alive. This would have caused an unintentional resource leak of threads/tasks. This was found by some internal profiling in search of #42.
Migrates to the new version of tokio, requiring channel and sleep changes in a few locations. Additionally points to the in-tree v0.3 version of twilight.
This change is made with queue users in mind. Since sources
of this kind *know* how to (re)create themselves, they can
avoid being created at all until needed.
This also adds machinery to preload tracks *before* they are
needed, for gapless playback on queues and so on. Queues
make use of the event system to do this.
* 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.
