Rejigger modal_map to properly handle ascending vs. descending motions
and ascending vs. descending interval steps, depending on what is going
on. Various internal changes to better support this. Differentiate
between linking pitches of the modal map and start pitch of the input
phrase (so can link C major to C major but start a phrase on anything
besides C major without also changing the mode).
Do not need _asym as internally ordering all interval sets tonic up, not
the other way for descending scales (that way lies madness).