Allow more argument types, interpret optional argument predicates syntactically.

Allow more argument types, interpret optional argument predicates syntactically.

This is a patch series also available as the dev/syntax branch instead
of a big patch.  It is intended for timely inclusion into Lilypond.
The patches are:

parser.yy: Allow a few more syntactic classes for Scheme arguments


programming-interface.itely: explain extended function argument semantics


parser.yy: Allow markups, markup lists, and context modifications as Scheme arguments


Add regtest for skipping optional arguments via predicate.


parser.yy: allow optional Scheme arguments to be skipped based on the predicate.


parser.yy: allow music rather than closed_music in connection with lyrics


parser.yy: let closed_music accept a few complex forms


parser: change order of arguments for context-specification constructor


parser.yy et al: move functionality of run_music_function into syntax constructor