v2.31.0b1

• Split kerning by script, not direction (#679). This is a rework of #636 and should now properly split up kerning into per-script lookups without losing any. The major upside is that multi-script fonts with a lot of kerning will compile faster and smaller, because lookups will overflow less often and become denser.
• As a side-effect, kerning will now also be generated such that the shaper will actually apply it when the designer did not explicitly declare an appropriate languagesystem in the font's features. Before, kerning for Greek glyphs was effectively dropped unless the designer declared languagesystem grek dflt;.
• Another side-effect is that kerning between some common glyphs like increment and asterisk will now always be applied, where before due to circumstances it might have been stuck in e.g. the LTR instead of the Common lookup and therefore not applied if the script wasn't set correctly.