[subset] Caclculate head table checksum adjustment

Test still fails, because we do not serialize tables in the same
order that fonttools subsetter does.

[subset] Fix several memory leaks in subsetting.

Also, call c++filt on their results.

Also build and fix and check harfbuzz-gobject.def

Also changes harfbuzz-gobject to distribute generated enum sources.

[subset] Add hdmx subsetting.

[subset] Fix checksum clearning in subset test diff. Add an updated expected file (has cmap4 restored).

More readable.

Fixes https://github.com/harfbuzz/harfbuzz/issues/817