TRE: Add local modifications to extend functionality

TRE: Add local modifications to extend functionality

The stock TRE regex library is very good, but it is missing three
key functionalities:
  1) collation support
  2) equivalence classes
  3) legacy support for character-name table

Luckily, TRE was imported in Apple's Libc and they solved these issues.
This commit represents the modifications Apple made (under the same
2-clause license the author Ville Laurikari issued) minus differences
in xlocale support and the exclusion of "if 0" equivalent blocks.