Experimental tweaks to how parallax's movement hooks work; the current ones appear to be fairly expensive for all movable movement, doing locate(/mob) in src on every forceMove(). This PR creates a new lazylist (contained_mobs) containing a list of every mob directly contained by an object. contained_mobs is updated on mob/forceMove().
Not sure how to handle mobs located deeper than directly inside an object.
Only alternative to this I see is to make SSparallax tick.