make nets keep list of updated gates instead of ports

sim/utility.lua

@@ -77,3 +77,11 @@ function array_remove(array, value)
 	end
 	error("element not in array")
 end
+
+function array_add(array, value)
+	for i = 1, #array do
+		local v = array[i]
+		if v==value then return end
+	end
+	table.insert(array, value)
+end