Use a small Box interface instead of a full Thing for collision detection

[JoshuaKGoldberg]

Packages that deal with the physics engine (i.e. QuadsKeepr and ThingHittr) take in full IThings and look at direct members such as .left. It'd be better to move space-related things in the IThing interface to a member, like box: IBox. At its simplest it would probably look something like:

interface IBox {
    top: number;
    right: number;
    bottom: number;
    left: number;
}

This might be dependent on #189 so that the box doesn't require offsets (so that things just barely not touching each other's quadrants can still collision detect).

Open question: should this box contain xvel and yvel? If so, is "box" the right term? As with Thing->Actor, what does everyone else call this thing?