Ports the Port of Circuits

Ports PsiOmegaDelta's port of integrated circuits, which has several improvements in code quality.
Ports a few small things like the weakref datum and some macros.

code/datums/weakref.dm

@@ -0,0 +1,31 @@
+/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(D.gcDestroyed)
+		return
+	if(!D.weakref)
+		D.weakref = new/weakref(D)
+	return D.weakref
+
+/weakref
+	var/ref
+
+/weakref/New(datum/D)
+	ref = "\ref[D]"
+
+/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
+
+/weakref/proc/resolve()
+	var/datum/D = locate(ref)
+	if(D && D.weakref == src)
+		return D
+	return null