* lily/system.cc (do_derived_mark): don't mark from object_alist_

* lily/system.cc (do_derived_mark): don't mark from object_alist_
anymore, but do it centrally.  Speedup: approximately 3-5 %.

* ly/engraver-init.ly (AncientRemoveEmptyStaffContext): remove
hammer hack.

* lily/grob-scheme.cc (LY_DEFINE): new function ly:grob-object

* scm/output-lib.scm: remove hammer-print-function.

* lily/include/pointer-group-interface.hh (extract_grob_set): new
macro. Declare a Link_array<Grob> and fill it from a grob.
(extract_item_set): idem for item.

* lily/break-substitution.cc: add header.
(fast_substitute_grob_array): rewrite for Grob_arrays.
(substitute_grob_array): idem.

* lily/group-interface.cc (add_thing): remove file.

* flower/include/parray.hh (class Link_array): slice() is const.

* lily/include/grob-array.hh: new file.

* lily/grob-array.cc (spanner): new file.

* lily/beam-quanting.cc (fill): read details property from beam.

* lily/beam.cc: support details property.

* lily/include/beam.hh: new struct, softcode beam quanting parameters

* lily/include/grob.hh (class Grob): add interfaces_ member.

* lily/bezier.cc (init_polynomial_cache): new function: cache
binom(3,j) t^j (1-t)^{3-j}
(curve_point): opps, actually use the cache for t^j , (1-t)^j!

* lily/grob-property.cc (internal_get_object): new routine.
(internal_set_object): idem. Store grob refrences in separate
alist. This saves processing time, since properties aren't
break-substituted, and the per grob namespace is smaller, both for
grobs and non-grob properties.

* scm/define-grob-properties.scm (all-internal-grob-properties):
remove center-element.

* lily/grob.cc: remove tweak-count, tweak-rank.