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

In the hb-subset to fontTools comparison, use ttx to compare the fonts. This allows for some binary differences such as re-ordered tables.

Comment out failing assert in test-subset-glyf for now. Should be re-enabled once hb_subset is writing out a new face.

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

Fixes https://bugs.chromium.org/p/chromium/issues/detail?id=657466

Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=5584