Allow hook for compare function + Ignore order about treediff-rs

[Byron]

Thanks for the detailed write-up, I believe this makes it easy for others to pick up if there is demand.

Indeed the algorithm itself makes certain assumptions and can't be affected by the passed in delegate.

The only way to fix this would be to have entirely different diff-functions, which I believe you could implement yourself based on your needs.

[flip111]

Well i have the suspicion it's not so much the algorithm as it's the chosen data structure. I read here:

https://doc.rust-lang.org/std/collections/struct.BTreeSet.html

It is a logic error for an item to be modified in such a way that the item's ordering relative to any other item, as determined by the Ord trait, changes while it is in the set.

Maybe it's possible to change the diff function to make it generic over a type so people can choose a structure that cares about order or not. I don't know if there is a candidate data structure that doesn't care about order, but i could look or ask around.