Merge duplicate numbers on multi-measure rests
Add a new
Merge_mmrest_numbers_engraver that is modeled after the
Merge_rests_engraver, but enabled by default. If a staff
has at least two multi-measure rest numbers with the same text, only
the first one is retained.
Fixes #6028 (closed) and resolves all collisions in rest-positioning.ly
Also simplifies the handling of multi-measure rests in
Merge_rests_engraver that I used as a template.