Makes way for adding arbitrary user_data support.

So we can liberally use the simple features of C++ that parts of the
codebase is already using.

Also optimizes the common-direction script lookup to be an array lookup.

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.