make sim use proper OOP

sim/wire.lua

@@ -21,7 +21,7 @@ function Wire.setlayer(self, layer)
 		Group.removewire(self.group, self)
 	end
 	self.layer = layer
-	Simulation.connectwire(self.sim, self)
+	Simulation.connectwire(Wire.getsim(self), self)
 end
 
 function Wire.update(self)
@@ -47,3 +47,7 @@ end
 function Wire.getbounds(self)
 	return self.bounds
 end
+
+function Wire.getsim(wire)
+	return wire.sim
+end