sanitizer.sanitize never return nullptr.

test-subset-glyf still failing. No idea why :(

Hopefully most bots come back with this...

Return empty face on hb_subset failure instead of null. Plus some minor cleanups for _add_head_and_set_loca_version

sketch a subset<T> and call it for cmap. Add subset to cmap, albeit not working even for the msot basic case just yet

test-subset fails now because subset-face does not know how to compile itself.

It's a hb_face_t that has add_table() and in the future knows how to
compile itself into a font blob.

We keep a separate private NullPool in libharfbuzz-subset.so
This can fire back later since now there are two separate null objects,
and comparisons to the null pool can be confused.  Something to deal
with when it comes up. Ouch!