- Add a standard set of extra compositions that are useful for font-building,
   such as characters WITH STROKE.
 - Support more combining classes.
 - Support for charcell/monospaced fonts. This would involve ignoring
   decompositions with spacing characters (Dz -> "D", "z"), or maybe just
   ignoring any decompositions that went outside the defined character cell.
 - Font manipulation functions should return a new font with the appropriate
   modifications rather than changing the original font in-place, so that
   "pipelines" of font modifications can be built.
 - Add a test for model.Font.__delitem__()