GSUB/GPOS subsetting

[behdad]

• Clean up UnsizedArray type and use it everywhere.
• Implement population caching in hb_set_t.
• Implement hb_auto_t<> that calls init/fini on the object inside, for auto lifecycle management.
• Write down Supplier protocol. A Supplier will have iter_forward() and iter_backward(). Check for C++ idioms.
• Write sized_array_t<> that takes unsized array-like object and a len and enforces bound checking.
• Write SubsetView<> that takes a set and an array-like object (ensure it has .len) and provides iterators for a subsetted view of the array.
• Change/implement serialize() methods that take a typename Supplier and work with it.
• Implement subset() methods that build SubsetView<>s and calls serialize().

[behdad]

This is done now. We have a lot of refactoring / cleanup to do. But that's separate.