Add compose() and decompose() unicode funcs.  These implement
pair-wise canonical composition/decomposition.

The glib/icu implementations are lacking for now.  We are adding
API for this to glib, but I cannot find any useful API in ICU.
May end of implementing these in-house.

Changed all unicode_funcs callback names to remove the "_get" part.
Eg, hb_unicode_get_script_func_t is now hb_unicode_script_func_t,
and hb_unicode_get_script() is hb_unicode_script() now.

In preparation for adding more advanced unicode funcs.

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.

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.

- Rename HB_SCRIPT_INVALID_CODE to HB_SCRIPT_INVALID

- Add HB_DIRECTION_INVALID

- Make hb_script_get_horizontal_direction() public

- Make hb_shape() guess script from buffer text (first non-common
  non-inherit script) if buffer script is set to HB_SCRIPT_INVALID (this
  is NOT the default.)

- Make hb_shape() guess direction from buffer script if buffer direction
is set to HB_DIRECTION_INVALID (this is NOT the default.)

- Make hb-view.c set INVALID script and direction on the buffer.

The above changes are meant to make hb-view fairly useful for uni-script
uni-direction text.  The guessing behavior however is NOT the default of
hb_shape() and must be asked for explicitly.  This is intended, because
the guess is not a suitable substitute to full-fledged bidi and script
segmentation.  It's just a testing tool.

So we can apply it to class methods also.  Not sure if that works!

This reverts commit c939f6aff405ca7b10b1f1538f46148bff719fcb.