add net and wire struct; move net update into its file

sim/simulation.lua

@@ -266,15 +266,7 @@ end
 
 function Simulation.tick(self)
 	for k, group in pairs(self.groupqueue) do
-		local newstate = false
-		for j, port in pairs(group.out_ports) do
-			newstate = newstate or Port.getstate(port)
-			if newstate then
-				break
-			end
-		end
-		
-		Group.setstate(group, newstate)
+		Group.update(group)
 	end
 	self.groupqueue = {}