This PR does the following:
* Changes both `Reader::Extension` and `Reader::ExtensionSeek` to use `symphonia::io::MediaSource`.
* Removes the `File` and `Vec` variants of readers, instead opting to provide a `from_file` and `from_memory` associated function to create readers from the `File` and `Cursor<Vec<u8>>` implementations of `MediaSource`.
* Removes the ReadSeek trait.
* Added a dependency on `symphonia_core`. This crate has no additional dependencies.
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.
Far cleaner and more reliable than the old doc-link pattern. Also allowed me to spot some event types and sources which should have been made non_exhaustive.
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.
