[subset] Add missing face reference in hb-subset-plan plus ensure all struct members are cleaned up on destroy.

Trying to fix this warning:
cc1: warning: command line option ‘-fno-threadsafe-statics’ is valid for C++/ObjC++ but not for C

Let's see what the clang bots think of it.

Switch from a 128+64 hierarchy, to 32+32. New page size is 1024.
This should make set iteration faster, without affecting other op
performances much.

We want to use it.

New API:
- hb_set_previous()
- hb_set_previous_range()

New API:
- hb_blob_copy_writable_or_fail()

To signify that unlike rest of library, returns nullptr on failure.