* scm/ly-from-scheme.scm (read-lily-expression): A variable

* scm/ly-from-scheme.scm (read-lily-expression):  A variable
refering to a music expression can be used in lily-inside-scheme:
#{ $music #}

* lily/my-lily-parser.cc (LY_DEFINE): introduce ly:clone-parser
and ly:parser-define, and change ly:parser-parse-string in order
to make #{ $music #} work.

* scm/new-markup.scm (compile-markup-expression): when an argument
is a string, use `make-simple-markup'.