* input/regression/grace-unfold-repeat.ly: new file
* lily/auto-beam-engraver.cc (test_moment): never start on grace moments.
* lily/forbid-break-engraver.cc (start_translation_timestep): skip
grace notes for break-forbidding. Breaks during grace notes are
handled by Bar_engraver.
* lily/unfolded-repeat-iterator.cc: Reimplement
Unfolded_repeat_iterator, Volta_repeat_iterator using
Sequential_iterator. All grace functionality from {} carries over
to repeats as well.
* lily/sequential-iterator.cc: new file: form baseclass for
sequential music iterator and unfolded/volta repeat iterator.