Implement Expansion Board serial ring and test ports

[gardners]

Is your feature request related to a problem? Please describe.
The MEGA65 Expansion board I am designing has user port and tape port and C1565 port that are all connected to some degree using a ring of serial shift buffers. We need to implement this ring, and test it with the board.

Describe the solution you'd like
I implement the ring, test it under simulation and with real hardware, to confirm it works.

Describe alternatives you've considered
Sleeping in

Additional context
N/A

[gardners]

See also https://github.com/mega65/mega65-r3-expansion-board