Issue 3531: replaced function argument 'string' by 'const string&' where it makes...

Issue 3531: replaced function argument 'string' by 'const string&' where it makes sense to avoid unnecessary copying of 'string' objects.

Measurements on x86_64 (i7-2760QM, 2.40GHz) Fedora 19 with g++ 4.8.1, with
configure --enable-optimising --disable-debugging; tests run 10 times, average
elapsed time compared (/usr/bin/time)

* Bach, Concerto in E major or violin and strings, BWV 1042 (Mutopia source),
  38 pages:
  $ lilypond score.ly -> master: 15.4s, with patch: -0.1%
* lilypond regression tests (1153 .ly files):
  $ lilypond *.ly -> master: 276.6s, with patch: -2.5%