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

This reverts commit 66920a6b.

Some of our bots (Oracle Studio and Apple gcc 4.2) do not allow
explicit except for constructors.

https://github.com/harfbuzz/harfbuzz/issues/1127

Was creating object even if wasn't there.

Found and fixed a couple bugs.

Found a couple multithreading issues.  Marked them with "XXX-MT-bug".

Removes volatile from fallback implementation.  That was handwavy
anyway.

If ever int is not 32bit, we'll fix this...

https://bugzilla.mozilla.org/show_bug.cgi?id=1476334

New API:
+hb_font_get_glyph_advances_for_direction

New API:
+hb_font_get_glyph_h_advances
+hb_font_get_glyph_v_advances

We would need to write:

  for (Iter<T> it (...); bool (t); t++)

instead of:

  for (Iter<T> it (...); t; t++)

But I think it's an improvement in code readability in the long term.

Fixes https://github.com/harfbuzz/harfbuzz/issues/1127

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.

New API:
+hb_font_funcs_set_glyph_h_advances_func
+hb_font_funcs_set_glyph_v_advances_func
+hb_font_get_glyph_h_advances
+hb_font_get_glyph_h_advances_func_t
+hb_font_get_glyph_v_advances
+hb_font_get_glyph_v_advances_func_t

Finally, unified!

The code was badly broken.  In better shape now, but still, needs
a full review before ever working.