Also optimizes the common-direction script 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.

Returns the error status of the buffer.

Return the length, whenever we return an array.  Makes it easier on the
language bindings.

This simplifies the code, reduces static data, and makes the design
more extensible to future additions of new scripts.

This includes HB_DIRECTION_INVALID and HB_SCRIPT_INVALID.

The INVALID will cause a "guess whatever from the text" in hb_shape().
While it's not ideal, it works better than the previous defaults at
least (HB_DIRECTION_LTR and HB_SCRIPT_COMMON).

One should use hb_buffer_reset() really.

I really shall find a way to automate the test enumeration.  Otherwise
it's too easy to add a test and wrongly hook it up.  Did it twice today.

It's just sugar.

The problem with HB_TAG_STR() was that it expected a string of size 4
exactly, and unlike hb_tag_from_string() it doesn't pad the tag with
space characters.  So, the new name is more appropriate.

Uses the glib testing framework.  Wrote unit tests for hb-common.h
types.