- There is now just one "playing" state.
- Instead of rewind, use :seek(0).
Note that now there is no way to resume or rewind all tracked sources.
This can be improved in the future if there's a need for it, probably
using variadic or table-based variants of the audio module functions.
- Ref struct only stores refcount now and is more general.
- Proxy stores a hash of its type name instead of an enum.
- Variants store additional information instead of using a vtable.
- Remove the concept of superclasses from the API.
- Clean up some miscellaneous includes.