Split kerning by script, not by direction (third attempt)

[madig]

This is a clean-up PR for #667.

This implements:

• Making classifyGlyphs also deal with tuples instead of just lists and sets.
• Making the KernFeatureWriter emit scripts independently of the globally declared languagesystems. This is arguably a separate concern. I removed some tests that no longer make sense.
  • The code now looks at the declared languagesystems and the font's codepoints that have just one script associated with them, as a heuristic for what the font is supposed to support. We don't depend on the designer to add all desired script declarations.
• Having KerningPair deal with just pair information, and deal with scripts and bidi metadata outside. This simplifies the code a lot by collecting concerns in one place.
• A method of not triggering erroneous subtable breaks in otlLib (swallowing kerning) by:
  • Using a simpler kern splitter that passes kerning validation and produces no overlapping kerning groups
  • Sorting the kerning pairs (glyph-to-glyph first, then glyph-to-class, class-to-glyph and class-to-class)
• Some more tests to test bidi and script splitting
• Dropping kerning classes from being written to the feature file
• Various code simplifications of the KernFeatureWriter.

Note that for reasons I have not investigated, various Noto Serif Tamil sources have some errant kerning, but they have the same problem with the current mainline kern writer.

Closes #658

[anthrotype]

mostly LGTM, thanks!

One thing before I think we can merge is, we should try to keep using named FEA classes in the generated feature code, either for the original classes that needed no split and for the split ones that occurred more than once; having no named classes (repeatedly spelling out all the [a b c] and such) makes the feature code harder to debug; and the alleged feaLib bug whereby some pairs may get lost if one mixes named and unnamed classes isn't sufficient to warrant ditching the named classes altogether, and we need to fix that on its own.

[anthrotype]

while reading old issues, I stumbled on a long comment of mine that details the way the ufo2ft kernFeatureWriter worked:

unified-font-object/ufo-spec#16 (comment)

It would be nice to have a similar write up for the newly porposed split-by-scripts algorithm..

[anthrotype]

LGTM!

I propose we merge and cut a pre-release and ask users to test this with their fonts to prevent the need to revert this once again.

Thanks a lot to both Simon and Nikolaus for working on this!