Missing formatter functor overload for u32regex_replace

This is an old ticket from the mailing list that never got any attention: https://lists.boost.org/boost-bugs/2017/09/48948.php

It also appeared on stackoverflow: https://stackoverflow.com/questions/29053141/boost-regex-no-prototype-for-u32regex-replace-with-callback-function

Start by reading the linked mailing-list ticket and Stack Overflow question, then search the Boost.Regex sources for u32regex_replace declarations and existing formatter overloads. Done means the missing formatter functor overload is supported and its callback use is covered by an appropriate test; the issue names no files or tests to begin with.

Written by the indexing model from the issue text.