"CFF" is used for shared structs & sources between CFF1 & CFF2
except OT:cff is for CFF version 1 table

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