Port of @PsiOmegaDelta's Baystation12/Baystation12#16820

Ports tg's garbage collector subsystem and Destroy() returning qdel hints.

code/datums/helper_datums/global_iterator.dm

@@ -155,4 +155,5 @@ Data storage vars:
 	tag = null
 	arg_list.Cut()
 	stop()
+	return QDEL_HINT_LETMELIVE
 	//Do not call ..()

code/datums/repositories/decls.dm

@@ -36,4 +36,4 @@
 
 /decls/Destroy()
 	crash_with("Prevented attempt to delete a decl instance: [log_info_line(src)]")
-	return 1 // Prevents Decl destruction
+	return QDEL_HINT_LETMELIVE // Prevents Decl destruction

code/datums/weakref.dm

@@ -7,7 +7,9 @@
 
 //obtain a weak reference to a datum
 /proc/weakref(datum/D)
-	if(D.gcDestroyed)
+	if(!istype(D))
+		return
+	if(QDELETED(D))
 		return
 	if(!D.weakref)
 		D.weakref = new/weakref(D)
@@ -22,7 +24,7 @@
 /weakref/Destroy()
 	// A weakref datum should not be manually destroyed as it is a shared resource,
 	//  rather it should be automatically collected by the BYOND GC when all references are gone.
-	return 0
+	return QDEL_HINT_LETMELIVE
 
 /weakref/proc/resolve()
 	var/datum/D = locate(ref)