Main goal: a lot of nested future/result folding.
This mainly modifies error handling for Tracks and TrackHandles to be
more consistent, and hides the underlying channel result passing in
get_info. Errors returned should be far clearer, and are domain
specific rather than falling back to a very opaque use of the underlying
channel error. It should be clearer to users why their handle commands
failed, or why they can't make a ytdl track loop or similar.
Also fixed/cleaned up Songbird::join(_gateway) to return in a single
await, sparing the user from the underlying channel details and repeated
Errs. I was trying for some time to extend the same graces to `Call`,
but could not figure out a sane way to get a 'static version of the
first future in the chain (i.e., the gateway send) so that the whole
thing could happen after dropping the lock around the Call. I really
wanted to fix this to happen as a single folded await too, but I think
this might need some crazy hack or redesign.
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.
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.
