Rework movement to prep for flexible player state machine

[nfagerlund]

That Maddy code I adapted is really meant for an OO paradigm, and some of its assumptions get a lil goofy in ECS land. I'd really like to factor the movement and collision out into a function I can call from multiple player states, and return collision info, like we did in the shaunjs tutorial... but I probably need to rethink some of those assumptions to do that!

(Needless to say, the GML version from the tutorial is even less suited to Rust world.)

[nfagerlund]

This is pretty much dealt with, see #4.