It was a nice idea to explicitly pass state to everything,
but it didn't really provide any benefits (we only ever have one
game going at once), the rule was already violated (some places
used the global instance of state), and it added a lot of clutter.
Also removed the unused entity.js file.

The passive movement is now cyclic, rather than completely random.
Swallows, slurps, and player movements are applied over a period
of time.

Slurps now come before chews, and there is a limit to both

You have to struggle forward to avoid being swallowed, but also can't go too far forward,
or else you'll be chewed.