collate: Add support for equivalence classes and collating symbols

This change adds support for matching single and multi-character
equivalence classes and collating symbols, as specified
by POSIX1, section 9.3.5.

Sponsored by: Klara, Inc.
Obtained from: https://github.com/apple-oss-distributions/Libc
Differential Revision: https://reviews.freebsd.org/D49659
Reviewed by: markj