tonos

Copperbox revision 1302.

I've added a Num instance for PitchLetters (as they are just Z7 integers in disguise) and a Distance type class which should model a metric space, except perhaps the triangle equality rule - I don't know how it applies to PitchLetters.

PitchLetters have a distance according to their semitone count, e.g. C to D is 2, but B to C is 1 (as considering a piano there is no black key between them).