bug#31138: Native json slower than json.el

[Eli Zaretskii]

> From: yyoncho <yyoncho@gmail.com>
> Date: Tue, 23 Apr 2019 18:36:18 +0300
> Cc: Philipp Stephani <p.stephani2@gmail.com>, Dmitry Gutov 
> <dgutov@yandex.ru>, 
>       Sébastien Chapuis <sebastien@chapu.is>, 
>       31138@debbugs.gnu.org
> 
> Another posibility is to have two string convertion methods.

We already have them, that's what json_make_string uses internally.
Right?