Simplify hb_utf_prev<8> to call hb_utf_next<8>
Simplify hb_in_range()

It's both faster and produces smaller code.  Now I feel stupid for
not writing it this way before.
Mark unsigned integer literals with the u suffix

Simplifies hb_in_range() calls as the type can be inferred.
The rest is obsessiveness, I admit.
Use hb_in_range() for arabic and indic tables

Though, looks like gcc was smart enough to produce the same code
before...
Simplify hb_utf_prev<16> to call hb_utf_next<16>
Only accept well-formed UTF-8 sequences

Enable tests that were disabled before, and adjust one test,
and add more tests.
Simplify / speed up UTF-8 code