Optimizes note-heads.cc and introduces robust_symbol2string. (issue 5233042)

[bordage . bertrand]

Reviewers: ,

Message:
Hi,

This is a try to increase performance, especially under Windows (see
http://code.google.com/p/lilypond/issues/detail?id=1926).

As I don't use Windows to compile LilyPond, can someone test it?

Regards,
Bertrand

Description:
Optimizes note-heads.cc and introduces robust_symbol2string.

Potential fix to issue 1926.

Please review this at http://codereview.appspot.com/5233042/

Affected files:
  M lily/include/lily-guile.hh
  M lily/lily-guile.cc
  M lily/lily-parser-scheme.cc
  M lily/note-head.cc
  M lily/page-turn-engraver.cc
  M lily/paper-column-engraver.cc
  M lily/program-option-scheme.cc
  M lily/rest.cc
  M lily/stem.cc
  M lily/time-signature.cc