Collision group logic is very messy

[mwillsey]

Collision group logic is very messy, needlessly complicating all sorts of things.

It introduces unnecessary mutability:

puddle/src/core/src/command.rs

Lines 29 to 30 in e8ea2f9

	// FIXME this shouldn't be mut, but we need to set the collision groups in mix
	fn dynamic_info(&self, &mut GridView) -> DynamicCommandInfo;

It also just makes the routing waaaay more complicated (and probably) buggy. We could fix this by having some cleaner (non-mutating) way of asking the router to allow things to collide. This would also be cleaner because we will actually know when the mix occurs, allowing us to update the droplet.