Factored out CFF1 & CFF2 common subsetting code in hb-subset-cff-common.hh

Subr-flattened charstrings are temporarily re-encoded in ByteStrBuff during "plan" phase, then copied to hb_serialize_context_t during "write" phase

CSOpSet may callback opcode processing "virtual" functions via CRTP

Numer struct may store a value as fixed optionally in addition to int and float

along with CFF2 charstring interpreter
factored out common code between CFF1 & CFF2 to CSInterpreter
moved fetch_op from Interpreter to InterpEnv
misc code clean up & bug fixes

to avoid confusion with OT::Index

Unreferenced subroutines are nullified (not removed) in a subset font

Separate Interpreter struct from hb-ot-cff-common-private.hh in a set of four header files, augmented for CharString (CS):
hb-cff-interp-common-private.hh
hb-cff-interp-dict-common-private.hh
hb-cff-interp-cs-common-private.hh
hb-cff-interp-cs.hh

Interpreter runtime is separated off as a new struct InterpEnv sub-classed differently for Dict and CharString (CS)

It was bloating a multi-FD subset font unnecessarily
defined a wrapper struct FDMap for fdmap

Added sources hb-ot-cff-table.hh, hb-subset-cff.cc & hh
Templatized Index because CFF uses 16-bit count while CFF2 uses 32-bit
Misc code cleanup & bug fixes

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

also fixed local subrs serialization

Added serialize functions to CFF2 structs
Fixed issues & bugs & code cleanup
Now subsetting CFF2 table outputs something

Added sanitize functions to FDSelect
Added satnitize calls for FDSelect, VariationStore, FDArray, TopDict

Index, Dict structs
hooked up to hb-subset (takes CFF2, outputs empty CFF2)