[core] Correcting discrepancies between current_transform() and matches

[Yey007]

Jess has been working on a new match display. Thanks to her work, we've discovered a number of bugs in how matches work right now. Due to the way iterate works, matches is usually one iteration behind current_transform() (except right after begin()) and thus functions like calculate_cost() are technically wrong. I don't know the best way to correct this right now, but it needs to happen.

[Yey007]

One option: force implementing classes to have a method that updates matches. Call it when necessary. Leads to unnecessary calls when using calculate_cost in a tight loop, though. Can potentially be rectified by keeping track of when things are stale? This seems annoying. Will get back to it later.