Fix scherzo to use pitch_t rather than score_note_t for computation

The score_note_t type is constructed as a side effect of performing
drawing. This is a poor fit for scherzo's computation and reasoning
since it often wants to perform analysis on notes that it has never
drawn.

Meanwhile, the new pitch_t is a simple integer, making it very easy
for scherzo to use pitch_t in all of its calculations.

The goal is for no functional changes with this commit, but we know
that some bugs have likely been changed or moved around.