Added a set of sources hb-subset-cff-common-private.cc & .hh for FDSelect subseting code.
Added FDSelect format 4 (CFF2 only) support. Shared its implementation with format 3 as a template.
Instead moved & stored uniformly in values vector (renamed from opstrs)
This should be fine as we aren't likely going to use them as hints after all
Templatized DictValues whether to parse them out
For subsetter, they aren't parsed