bug#31138: Native json slower than json.el

[Eli Zaretskii]

> Date: Tue, 23 Apr 2019 18:09:55 +0300
> From: Eli Zaretskii <eliz@gnu.org>
> Cc: sebastien@chapu.is, dgutov@yandex.ru, yyoncho@gmail.com,
>       31138@debbugs.gnu.org
> 
> If we want to make this more general-purpose, we should time the code
> with very long strings.  It could be that beyond some length the UTF-8
> verification, if it fails, will make the conversion slower, in which
> case we probably should not take the shortcut beyond that length
> limit.

Another minor issue to consider is that moving json_make_string into
another file might make JSON processing somewhat slower, because the
function will/might no longer be inlined.  Not sure if this is
important enough, though.  Perhaps timing the results will tell.