Fixes Stairs in BYOND 512.1436

BYOND no longer calls Uncross reliably on stairs, to fix this we switch
to using CheckExit and Bumped.  This solution is probably the right way to
do it anyway.

code/modules/multiz/structures.dm

@@ -141,20 +141,23 @@
 		if(!istype(above))
 			above.ChangeTurf(/turf/simulated/open)
 
-/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)
-			if(isliving(A))
-				var/mob/living/L = A
-				if(L.pulling)
-					L.pulling.forceMove(target)
-		return 0
-	return 1
+/obj/structure/stairs/CheckExit(atom/movable/mover as mob|obj, turf/target as turf)
+	if(get_dir(loc, target) == dir && upperStep(mover.loc))
+		return FALSE
+	. = ..()
+
+/obj/structure/stairs/Bumped(atom/movable/A)
+	// This is hackish but whatever.
+	var/turf/target = get_step(GetAbove(A), dir)
+	if(target.Enter(A, src)) // Pass src to be ignored to avoid infinate loop
+		A.forceMove(target)
+		if(isliving(A))
+			var/mob/living/L = A
+			if(L.pulling)
+				L.pulling.forceMove(target)
+
+/obj/structure/stairs/proc/upperStep(var/turf/T)
+	return (T == loc)
 
 /obj/structure/stairs/CanPass(obj/mover, turf/source, height, airflow)
 	return airflow || !density