Keyboard driver

Load range: 566-570

Implement a (key) word that interpret keystrokes from the
builtin keyboard. The word waits for a digit to be pressed and
returns the corresponding ASCII value.

This routine waits for a key to be pressed, but before that, it
waits for all keys to be de-pressed. It does that to ensure
that two calls to _wait only go through after two actual key
presses (otherwise, the user doesn't have enough time to
de-press the button before the next _wait routine registers the
same key press as a second one).

                                                        (cont.)