Optimisation: face selection for fallbacks

[dhardy]

Related: #53
Importance: probably low since harfbuzz/ttf-parser#68

Run-splitting must discover the font face to be used for each character. For this we use FontLibrary::face_for_char which iterates through all applicable faces, using Face::glyph_index on each, returning the FaceId of the first matching face (but discarding the GlyphId which is rediscovered during shaping).

There are a few possible optimisations here (but 2 and 3 conflict):

1. Batch look-ups. prepare_runs calls face_for_char_or_first for each char; it may be faster to iterate through chars for each subtable (see 0.12 to 0.13 performance regressions harfbuzz/ttf-parser#76). The same could be done in shape_simple.
2. Add a Face::contains_glyph method to ttf-parser (using the glyph_index method as a starting point, this should not be hard, but requires additional testing).
3. Cache both FaceId and GlyphId for use when shaping. This should be usable for shape_simple though probably not for Rustybuzz or Harfbuzz backends, so not particularly useful.