Refactor - Pulled reusable engine parts into a 'yape' folder

The diff is a bit startling, but looking here should make the layout a bit clearer:
https://github.com/calebsmith/razzytails/tree/refactor/engine-folder

I've basically moved fsm, dispatch, asset_loaders into the "engine" section. I've also pulled Screen out as its own module and put that into the engine.

@daaray and @vkurup: Feel free to take a look and LMK thoughts about where the changes so far.