This was a bizzare piece of API that I inherited from cairo. It has
been wrong adding them to cairo in the first place. Remove them before
someone uses them!
Unicode data providers can now be subclassed, including support for
chain-up. The interface should now be nicely bindable, as well.
Also fix glib unicode funcs that where broken after hb_script_t
changes. Nicely caught by the test-unicode.c added in this commit.