Redresses a previous holdover from an attempt to get Restartable sources to work more neatly inside the synchronous mixer thread. This prevents `Restartable::*` from blocking without warning.
The initial fix at the time was to perform the restart work on a task provided by the tokio runtime as `executor::block_on` needs to be run from within a valid async runtime. Naturally, this completely missed the point that these closures should/could be async, without any need to fudge async functions into a sync wrapper.
Also removes the `From` for normal closures, as this will probably act as a footgun for folks on a single-threaded executor.
* 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.
This implements a proof-of-concept for an improved audio frontend. The largest change is the introduction of events and event handling: both by time elapsed and by track events, such as ending or looping. Following on from this, the library now includes a basic, event-driven track queue system (which people seem to ask for unusually often). A new sample, `examples/13_voice_events`, demonstrates both the `TrackQueue` system and some basic events via the `~queue` and `~play_fade` commands.
Locks are removed from around the control of `Audio` objects, which should allow the backend to be moved to a more granular futures-based backend solution in a cleaner way.
