- a method to iterate the scripts + extensions for a codepoint
- ability to classifty glyphs into groups based on bidi class or &
  script

The opentype script tags have a different format from the short names
used by unicode.

This is very closely based on the code in the KernFeatureWriter, in
ufo2ft. With this patch we very nearly match fontmake's output for
kerning in oswald, with a few lingering differences.

It feels like there is room for polish here, but I also think it's worth
checkpointing here, for further iteration.