Clean up and improve the between-actors collision code:
* Give names to the A, B tuples from BetweenBoxes and call them the stable and
mover doodads. All ops are from the perspective of the stable (A) box in
relation to the mover (B).
* Replace the map[*Actor]*Actor collision struct to one that supports arrays of
colliding actors, enabling one actor to post OnLeave events correctly when it
overlapped several actors at once.
* Possibly improve on some of the collision bugs along the way.
Some issues remaining:
* If playing AS an offset-hitbox character:
* Solid hitboxes against closed doors and platforms works OK.
* Button InHitbox (overlap) checks do not fire (buttons won't press).
* If playing with a non-offset character (e.g. boy.doodad)
* Solid hitboxes work and Buttons will press correctly.
* Both types of player hitboxes:
* When touching a static object such as the Start Flag which has an offset
hitbox of its own: the Start Flag will creep up the level on its Y axis
when touched before it settles and won't move.