[Perl/perl5] 2599b9: sv.c: Use utf8_hop instead of rolling our own

[Karl Williamson via perl5-changes]

  Branch: refs/heads/blead
  Home:   https://github.com/Perl/perl5
  Commit: 2599b98cb9688b3eec0f4afae89d9f7c1f119243
      
https://github.com/Perl/perl5/commit/2599b98cb9688b3eec0f4afae89d9f7c1f119243
  Author: Karl Williamson <k...@cpan.org>
  Date:   2023-07-09 (Sun, 09 Jul 2023)

  Changed paths:
    M sv.c

  Log Message:
  -----------
  sv.c: Use utf8_hop instead of rolling our own

The inlined functions should be no slower than the hand rolling, and the
code is easier to maintain, with better handling of malformed input.