Rename Sink::empty

[dvdsk]

It should probably be called is_empty. Clippy even flags on it.

Changing it would break a lot of code though...
Could deprecate it first?

Let me hear you thoughts, for now I am letting clippy now to ignore it.

[dvdsk]

same is true for SpatialSink::empty

[daniellga]

Any improvements on this?

[dvdsk]

not really, I am saving up some breaking changes for a big release. I rather not break ppls code multiple times. So it seems logical to save it up and provide a porting guide.

[dvdsk]

though it might make sense to start collecting the changes on a new branch