bug#31138: Native json slower than json.el

[Eli Zaretskii]

> Cc: p.stephani2@gmail.com, sebastien@chapu.is, yyoncho@gmail.com,
>  31138@debbugs.gnu.org
> From: Dmitry Gutov <dgutov@yandex.ru>
> Date: Mon, 22 Apr 2019 19:16:15 +0300
> 
> All right. Does the attached json_encode_string_ascii_test.diff look 
> good to you?

The size == nchars case not necessarily true just for ASCII, I think,
and the code in parse_str_as_multibyte is more complex than needed
just for ASCII.  I meant make a loop that literally checks only for
ASCII_CHAR_P, and nothing more.  I think it makes sense to see first
how much can we speed up things for pure ASCII JSONs.

> +  if (size == nchars)
> +    /* Wasn't sure if the last arg should better be true here, but
> +       it doesn't seem to affect the behavior or correctness, so far. */
> +    return make_specified_string (data, -1, size, false);

It should make no difference for ASCII, but I think I prefer to use
'true' there, for consistency with the other cases.

Thanks.