new Serial robot doesn't track the port object, can't close port when closing robot

[JamesNewton]

The low level serial connect code adds the serial port object into the serial_path_to_info_map entry when it opens a port, and so when the low level close is called, the port can be closed.

The high level serial code does NOT save the port object, but then tries to use it to close the port when the serial robot is closed. This fails, and now no other robot can use that port because it is lost.

(writing this to remind myself to fix it tomorrow am. It's late and I just found this bug, too tired to fix it)