Driver, Tracks: Cleanup of leaky types (#20)

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.

src/driver/tasks/mixer.rs

@@ -322,7 +322,7 @@ impl Mixer {
             if temp_len > 0 || opus_len.is_some() {
                 track.step_frame();
             } else if track.do_loop() {
-                if let Some(time) = track.seek_time(Default::default()) {
+                if let Ok(time) = track.seek_time(Default::default()) {
                     // have to reproduce self.fire_event here
                     // to circumvent the borrow checker's lack of knowledge.
                     //