Port set_stat and COMSIG_MOB_STATCHANGE from TG

code/modules/mob/mob.dm

@@ -1009,3 +1009,15 @@ GLOBAL_VAR_INIT(exploit_warn_spam_prevention, 0)
   */
 /mob/proc/on_item_dropped(obj/item/I)
 	return
+
+/**
+ * Used to wrap stat setting to trigger on-stat-change functionality.
+ * Must be used instead of directly setting a mob's stat var,
+ * so that the signal is sent properly.
+ */
+/mob/proc/set_stat(new_stat)
+	if(new_stat == stat)
+		return
+	. = stat
+	stat = new_stat
+	SEND_SIGNAL(src, COMSIG_MOB_STATCHANGE, new_stat, .)