[DNM] Fixes all that Bump() nonsense

Created by: Exxion

How it works: Every instance of Cross() in the codebase was changed to CrossCheck(). This applies to both definitions and calls. On the /atom/movable level, Cross() is defined to call CrossCheck() and, if that returns false (and the object being crossed is dense and on a turf), the object being crossed moves itself to the beginning of its loc's contents. This causes it to be the object that gets bumped. Also, border objects are checked before anything else, to prevent that causing nonsense.

Problems:

1. I don't like the way I made it check border objects first. It works, but it's not exactly great code. I'll try to figure out a better way to do it.
2. Mobs are fucky. Apparently, they are handled differently from all other /atom/movables and cannot be manually inserted into contents, so they still don't work. I can't think of a way around this other than moving all OTHER dense objects to the END of contents when a mob should be bumped. That sounds like an awful solution and I'd rather not use it, but I will if nothing better comes up.