Add `TreeTraversalState::diff`

[kring]

Adds a diff method to TreeTraversalState. This method calls a callback function for each state instance that is different in the current traversal compared to the previous one.

The idea is that this allows us to report per-view-group tile state changes. We can use that to build a system that keeps points on terrain as a tileset refines/unrefines.

I think this is in pretty good shape, but I've marked it draft for now. The next parts we build on top of it might end up influencing the design.