* lily/lily-guile.cc (alist_to_hashq): new function

* lily/lily-guile.cc (alist_to_hashq): new function

* scm/lily.scm (alist->hash-table): new function.

* Documentation/user/refman.itely (Percussion staves): update doco.

* input/regression/drums.ly (timb): updates

* lily/note-performer.cc (create_audio_elements): robustness fix.

* ly/engraver-init.ly: add DrumStaff and DrumVoice contexts

* lily/drum-note-engraver.cc: new file.

* ly/drumpitch-init.ly: move drum definitions

* scm/drums.scm: remove file.

* lily/*.cc: remove abort-event everywhere.

* lily/note-heads-engraver.cc (process_music): robustification.

* lily/accidental-engraver.cc (process_acknowledged_grobs): robustification.

* lily/parser.yy (simple_element): add \drums mode.
(simple_element): remove \pitchnames , \chordmodifiers. Use
variables to store this.

* lily/lexer.ll (My_lily_lexer): remove support for \PITCHNAME
outside \notes.