baseFeatureWriter: add getOrderedGlyphSet method, etc.
baseFeatureWriter: add getOrderedGlyphSet method, etc.

Plus:

- when making the unicode to glyphName mapping, use the compiler.glyphSet
  if any (glyphs may have been pre-processed)

- don't compile temporary GSUB more than once, cache result; it is
  guaranteed not to change because all the GSUB feature writers are
  run first, then all the GPOS ones.

moyogo/ufo2ft,jamesgk/ufo2ft,jamesgk/ufo2fdk,googlefonts/ufo2ft,googlei18n/ufo2ft