* input/regression/grace-auto-beam.ly: new file

* input/regression/grace-auto-beam.ly: new file

* 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.