[introspection] Enable gobject-introspection again
[gobject] Flesh out hb-gobject

Move it to new harfbuzz-gobject library.  Implement enums and
boxed types for object-types.  Still have to do boxed types for
value types.
Hookup harfbuzz-gobject to introspection
Turn hb-gobject, and introspection off by default

Also enable bootstrapping without them.