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
> 
> +  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);
> +  else
> +    return code_convert_string (make_specified_string (data, -1, size, 
> false),
> +                                Qutf_8_unix, Qt, false, true, true);

Btw, we could somewhat speed up even the case where we do call
code_convert_string, by avoiding the call to make_specified_string.
That's because decode_coding_object, called by code_convert_string to
perform the decoding, is capable of decoding a C string, if we set up
its argument accordingly.  You can see one example of that in
w32select.c:w32-get-clipboard-data.