Rewriting multiz movement to be gooder (#2464)

Intent: implement all of the features from #2442 while also unfucking the multiz movement files to be more readable and not recurse.

To that end, this PR does the following:

All multiz travel is now arbitrated by SSfalling. This will eliminate the need for recursion without relying on timers. Timers would be a bit scary.
The call chain for multiz movement now looks like this (in terms of overwritable procs):
can_fall() (Can lead into fall_impact() & fall_collateral() if returns FALSE)
fall_through() IF current block is open space ELSE fall_impact() & fall_collateral()
Removed almost every istype(src, A) check in the movement.dm file by exercising proper parenting and call chains.
Documented and standardized the contents of multiz/movement.dm. Because this is an API we're going to be relying on very heavily, I'd like to get this right before we move on.
A lot of minor tweaks, like swapping usr over to the applicable mob, and so on.

code/modules/multiz/structures.dm

@@ -131,47 +131,47 @@
 	opacity = 0
 	anchored = 1
 
-	Initialize()
-		. = ..()
-		for(var/turf/turf in locs)
-			var/turf/simulated/open/above = GetAbove(turf)
-			if(!above)
-				warning("Stair created without level above: ([loc.x], [loc.y], [loc.z])")
-				return qdel(src)
-			if(!istype(above))
-				above.ChangeTurf(/turf/simulated/open)
+/obj/structure/stairs/Initialize()
+	. = ..()
+	for(var/turf/turf in locs)
+		var/turf/simulated/open/above = GetAbove(turf)
+		if(!above)
+			warning("Stair created without level above: ([loc.x], [loc.y], [loc.z])")
+			return qdel(src)
+		if(!istype(above))
+			above.ChangeTurf(/turf/simulated/open)
 
-	Uncross(atom/movable/A)
-		if(A.dir == dir)
-			// This is hackish but whatever.
-			var/turf/target = get_step(GetAbove(A), dir)
-			var/turf/source = A.loc
-			if(target.Enter(A, source))
-				A.loc = target
-				target.Entered(A, source)
-			return 0
-		return 1
+/obj/structure/stairs/Uncross(atom/movable/A)
+	if(A.dir == dir)
+		// This is hackish but whatever.
+		var/turf/target = get_step(GetAbove(A), dir)
+		var/turf/source = A.loc
+		if(target.Enter(A, source))
+			A.loc = target
+			target.Entered(A, source)
+		return FALSE
+	return TRUE
 
-	CanPass(obj/mover, turf/source, height, airflow)
-		return airflow || !density
+/obj/structure/stairs/CanPass(obj/mover, turf/source, height, airflow)
+	return airflow || !density
 
-	// type paths to make mapping easier.
-	north
-		dir = NORTH
-		bound_height = 64
-		bound_y = -32
-		pixel_y = -32
+// type paths to make mapping easier.
+/obj/structure/stairs/north
+	dir = NORTH
+	bound_height = 64
+	bound_y = -32
+	pixel_y = -32
 
-	south
-		dir = SOUTH
-		bound_height = 64
+/obj/structure/stairs/south
+	dir = SOUTH
+	bound_height = 64
 
-	east
-		dir = EAST
-		bound_width = 64
-		bound_x = -32
-		pixel_x = -32
+/obj/structure/stairs/east
+	dir = EAST
+	bound_width = 64
+	bound_x = -32
+	pixel_x = -32
 
-	west
-		dir = WEST
-		bound_width = 64
+/obj/structure/stairs/west
+	dir = WEST
+	bound_width = 64