Add object for multibit digital io (parallelio maybe?)

[tannewt]

This should allow for reading and writing values to multiple pins at once.

A shift register can then implement this same API too.

Once this is added we can cleanup GamePad back into one class.

[dhalbert]

Once this is added we can cleanup GamePad back into one class.

This part is now moot due to keypad.ShiftRegisterKeys, but the general idea is still interesting.

[dhalbert]

There are several ways to do this in various ports, so closing the general issue. Port-specific requests could be opened.