Contracts - Lotta Stuff

The first thing I did was implement contracts. Then I went through and changed the way that ID stuff is managed, and added a few helper functions to reduce the amount of redundant-but-critical code. So there are just a small handful of places where redundant code exists, and they are marked by comments.

I didn't add any new tests for the contracts. That'll be it's own PR b/c it's likely to be another 200-300 lines of code.

Once I'm done with that... the core stuff should be complete!