unicode

Currently hb_face_get_table_tags() only works with our own face loader. Other loaders cannot implement it. Add a callback setter such that they can. Then implement it for hb-ft.

Implement the following VT sequence:

CSI Ps SP u
       Set margin-bell volume (DECSMBV), VT520.
         Ps = 0 , 5 , 6 , 7 , or 8  -> high.
         Ps = 1  -> off.
         Ps = 2 , 3  or 4  -> low.

• Cleanup the tests
• Move module-specific tests into their own modules
• Rename the test case functions as we do in benchmarks

Currently it wraps an Rc, it should use an Arc if the sync feature is enabled.

This can probably reuse the RcWrap type from data_provider.rs, but we need to make it generic and plumb through additional methods like downcast.