Add basic scheme programmable engravers.

Add basic scheme programmable engravers.

* input/regression/scheme-engraver.ly shows a basic example.

* extend \consists syntax to accept an alist of callables.

* add Scheme_engraver which is the C++ glue to the Scheme callables.

* Make get_listener_ in translator_listener_record also pass the
  listened class, so we can use generic infrastructure for hooking
  scheme functions to event listeners.

* add scheme bindings:

  - ly:translator-context
  - ly:context-moment
  - ly:engraver-make-grob

* Remove Translator::must_be_last_.  Use virtual method must_be_last
  () const instead.