Remove field access from EmptyCallback

[dvdsk]

There is no reason why this:

rodio/src/source/empty_callback.rs

Line 10 in 70c236c

pub phantom_data: PhantomData<S>,

is public. Same for the callback. The struct should only be made by the factory function EmptyCallback::new.

[PetrGlad]

Should be done with #678