New markup commands \left-brace and \right-brace.

New markup commands \left-brace and \right-brace.

* add a binary search function to lily-library.scm

* add markup command \left-brace, using binary search to select a fetaBraces
  glyph based on y-extent comparison with scaled point size

* add markup command \right-brace: a \left-brace rotate 180 degrees

* add exported function ly:otf-glyph-count and use in \left-brace to remove
  hard-coded value for upper range in binary search

* fix potential null dereference in ly:otf-font-glyph-info

* add regression tests for exercising new commands and invalid point sizes