Issue 3665: transposing \chords does no longer work

Issue 3665: transposing \chords does no longer work

This consists of a number of commits (listed in reverse order below).

The first two commits (at the bottom of the list) are just general
cleanup to make the following stuff easier to see.  The next commit is
a bug fix: make_music_from_simple could return non-music in
\chordmode.  The next is a syntax change allowing pitch identifiers to
be followed by ' or , optionally, similar to how duration identifiers
can be followed by additional dots and/or factors, like \breve.*3/2
shows.  That makes the rest easier to do.

The actual problem fix consists of two prongs, one catering for
optional arguments, and the last (top of the list) catering for
mandatory arguments (or optional arguments at the end of the argument
list) in non-closed argument lists.  The greedy way in which the
latter are parsed (in particular accepting functions with non-closed
endings themselves) makes it unfeasible to do both with the same
approach.

Here is the list of commits in reverse order:

Let simple_music possibly return a pitch in \chordmode


Make sure optional arguments accepted as pitches stay pitches when reparsing


Let a PITCH_IDENTIFIER optionally be followed by octave modifiers

This makes pitch identifiers more similar to note names.  There is no
point in keeping the distinction just for triggering syntax errors
on music that has no sensible other interpretation.


Make make_music_from_simple return music rather than a list in \chordmode


parser: move FRACTION into embedded_scm_bare_arg

It's a closed expression; not having to parse it with separate rules
reduces the amount of cruft in the rules.


Fold function_arglist_backup_common into function_arglist_backup

After the last refactoring, function_arglist_backup does not need a closed
variant, so there is no point in an extra non-terminal.