Add weakrefs, because just two types of ref wasn't confusing enough (#3020)

Ports a new type of reference from bay: weakrefs.

They validate type like hard-refs, but don't block GC like soft-refs. Performance-wise are worse than both, but generally more convenient than softrefs.

code/controllers/subsystems/garbage.dm

@@ -227,6 +227,7 @@ var/datum/controller/subsystem/garbage_collector/SSgarbage
 // 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