Unify datum var definitions

Inspired by https://github.com/tgstation/tgstation/pull/29636
Also consolidated some sideways overridden /datum/Delete() here to reduce proc-call overhead.

code/datums/datum.dm

@@ -0,0 +1,22 @@
+//
+// datum defines!
+// Note: Adding vars to /datum adds a var to EVERYTHING! Don't go overboard.
+//
+
+/datum
+	var/gc_destroyed //Time when this object was destroyed.
+	var/weakref/weakref // Holder of weakref instance pointing to this datum
+
+#ifdef TESTING
+	var/tmp/running_find_references
+	var/tmp/last_find_references = 0
+#endif
+
+// 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 // Clear this reference to ensure it's kept for as brief duration as possible.
+	tag = null
+	nanomanager.close_uis(src)
+	return QDEL_HINT_QUEUE

code/datums/weakref.dm

@@ -1,10 +1,3 @@
-/datum
-	var/weakref/weakref
-
-/datum/Destroy()
-	weakref = null // Clear this reference to ensure it's kept for as brief duration as possible.
-	. = ..()
-
 //obtain a weak reference to a datum
 /proc/weakref(datum/D)
 	if(!istype(D))