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.