* lily/system.cc (spanner_count): new function

* lily/system.cc (spanner_count): new function

* lily/break-substitution.cc (fast_fubstitute_grob_list): special
function for break substitutions on groblists in spanners. By
reordering the elements of the list, we can skip large parts of
the list in the break substitution. This brings the complexity of
Lily back to more-or-less linear in the length of the score.