SMS pad

There is a driver for getting (key?) input from a SMS pad at
B335.

It conveniently exposes an API to read the status of a SMS pad
on port A. Moreover, implement a mechanism to input arbitrary
characters from it. It goes as follow:

* Direction pad select characters. Up/Down move by one,
  Left/Right move by 5
* Start acts like Return
* A acts like Backspace
* B changes "character class": lowercase, uppercase, numbers,
  special chars. The space character is the first among special
  chars.
* C confirms letter selection

This module needs CELL! (see doc/grid.txt) to display selection
on screen during (key?).

_status ( -- n )

Returns a status bitmask for port A. Bits, from MSB to LSB:
Start - A - C - B - Right - Left - Down - Up
Each bit is high when button is unpressed and low if button is
pressed. When no button is pressed, $ff is returned.
This logic below is for the Genesis controller, which is modal.
TH is an output pin that switches the meaning of TL and TR. When
TH is high (unselected), TL = Button B and TR = Button C. When
TH is low (selected), TL = Button A and TR = Start. )