Unify datum definitions (#3991)

Unifies all datum vars into a single file & moves all datum procs that made sense into that same file.

code/datums/datum.dm

@@ -0,0 +1,27 @@
+/datum
+	var/tmp/list/active_timers
+	var/tmp/datum/weakref/weakref
+	var/tmp/isprocessing = 0
+	var/tmp/gcDestroyed //Time when this object was destroyed.
+
+// Default implementation of clean-up code.
+// This should be overridden to remove all references pointing to the object being destroyed.
+// Return the appropriate QDEL_HINT; in most cases this is QDEL_HINT_QUEUE.
+/datum/proc/Destroy(force=FALSE)
+	weakref = null
+	destroyed_event.raise_event(src)
+	SSnanoui.close_uis(src)
+	tag = null
+	var/list/timers = active_timers
+	active_timers = null
+	if (timers)
+		for (var/thing in timers)
+			var/datum/timedevent/timer = thing
+			if (timer.spent)
+				continue
+			qdel(timer)
+
+	return QDEL_HINT_QUEUE
+
+/datum/proc/process()
+	return PROCESS_KILL

code/datums/weakref.dm

@@ -1,10 +1,6 @@
-/datum
-	var/datum/weakref/weakref
-
 /datum/weakref
 	var/ref
 
-
 /datum/weakref/New(datum/D)
 	ref = SOFTREF(D)