Tremolo cleanup. (issue 3745)

Tremolo cleanup. (issue 3745)

Scale durations when iterating, not in make-repeat. This means the original
repeat body is intact, so unfold-repeats and display methods no longer need
to undo the scaling.

Build tremolo spanners in Scheme, not C++.

Calculate tremolo-type in tremolo::get-music-list rather than storing it as a property.

shift-duration-log now clears the cached length, since it changes the length.

\shiftDurations no longer duplicates shift-duration-log.

Merge make-repeated-music with make-repeat.