bug#20154: 25.0.50; json-encode-string is too slow for large strings

[Eli Zaretskii]

> Date: Sat, 21 Mar 2015 09:58:43 +0200
> From: Eli Zaretskii <eliz@gnu.org>
> Cc: 20154@debbugs.gnu.org
> 
> So an alternative strategy would be to use 'skip-chars-forward' to
> skip to the next locus where encoding is necessary, and 'append' to
                                                          ^^^^^^^^
Sorry, I meant 'concat', of course.