Cleans up slipping code

Puts slipping into one proc rather than being dealt with individually by
each item.

The only changes to functionality here are where it would have been
possible to slip on some items whilst flying. It will make any changes
decided on in #3490 or any future changes far more easy to implement

code/modules/mob/living/carbon/carbon.dm

@@ -759,4 +759,24 @@ var/list/ventcrawl_machinery = list(/obj/machinery/atmospherics/unary/vent_pump,
 			W.loc = loc
 			W.dropped(src)
 			if (W)
-				W.layer = initial(W.layer)
+				W.layer = initial(W.layer)
+
+
+/mob/living/carbon/proc/slip(var/slippery, var/stun, var/weaken, var/tilesSlipped, var/walkSafely, var/slipAny)
+	if (flying || buckled || (walkSafely && m_intent == "walk"))
+		return
+	if (!(slipAny))
+		if (istype(src, /mob/living/carbon/human))
+			var/mob/living/carbon/human/H = src
+			if ((isobj(H.shoes) && H.shoes.flags & NOSLIP) || H.species.bodyflags & FEET_NOSLIP)
+				return
+	if (tilesSlipped)
+		for(var/t = 0, t<=tilesSlipped, t++)
+			spawn (t) step(src, src.dir)
+	stop_pulling()
+	src << "<span class='notice'>You slipped on the [slippery]!</span>"
+	playsound(src.loc, 'sound/misc/slip.ogg', 50, 1, -3)
+	if (stun)
+		Stun(stun)
+	Weaken(weaken)
+	return 1