Add `ps2_controller`: simplify PS/2 keyboard and mouse init (#700)

* The `ps2_controller` crate better represents the device hardware
  used to communicate with the PS/2 keyboard and mouse devices.

* Performs a fuller, more defensive init procedure with more testing
  and sanity checks, and fewer assumptions about the initial state
  of the PS/2 hardware.

Co-authored-by: Kevin Boos <kevinaboos@gmail.com> e8d9542538a387088ec3b108ed4724e7c5122479