[ttgxvar] Implement avar 2.0
I'm opening this to solicit feedback. avar 2.0 is a new version of the avar
table, that I spec'ed here for now:
https://github.com/be-fonts/boring-expansion-spec/issues/14
and HarfBuzz 5.0.0 supports. It solves various long-standing issues with variable fonts.