Delete difference?

[oxinabox]

The difference function is a first step towards #97.
However the rest of the steps for #97 haven't been done yet, and are not planned in the short-term.

Maintaining difference is kinda annoying,
and I suspect once we have the notion of projecting to different representations will want some redesigning.

It's not exported and it isn't used, so i think we can delete it as a nonbreaking change.

We can bring it back later when we need it.