Makes cable use BYOND directions

Instead of 11,12 they use 32,16 for DOWN,UP
Merges multiz handling into main logic
Port of: https://github.com/Baystation12/Baystation12/pull/15907

code/modules/multiz/basic.dm

@@ -43,3 +43,11 @@ var/z_levels = 0 // Each bit represents a connection between adjacent levels.  S
 
 proc/AreConnectedZLevels(var/zA, var/zB)
 	return zA == zB || (zB in GetConnectedZlevels(zA))
+
+/proc/get_zstep(ref, dir)
+	if(dir == UP)
+		. = GetAbove(ref)
+	else if (dir == DOWN)
+		. = GetBelow(ref)
+	else
+		. = get_step(ref, dir)