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Rewrite for Canon P23-DH V

Instead of tapping into the existing calculator circuit and trying
really hard not to interfere with its operation, while counting pulses,
when a button is pressed, we instead usurp half the surface area of
every button and construct our own matrix on that half.

Then, the pulse counting and all other garbage becomes unnecessary. We
only need to debounce the presses and the releases (cf, sleep(30)
timeouts).