bug#31138: Native json slower than json.el

[Eli Zaretskii]

> Cc: sebastien@chapu.is, yyoncho@gmail.com, 31138@debbugs.gnu.org
> From: Dmitry Gutov <dgutov@yandex.ru>
> Date: Wed, 24 Apr 2019 01:34:37 +0300
> 
> > Or we might document that non-UTF-8 strings will be slower to convert,
> > if that's acceptable.
> 
> *Invalid* UTF-8 strings. Strings that aren't actually UTF-8, even though 
> the caller declares them to be.

We are talking about a function that expects UTF-8 strings, so
"non-UTF-8" and "invalid UTF-8" are 2 different ways of saying the
same thing.

However, note that various 7-bit encodings will be taken by such an
interface as valid UTF-8, since all the bytes are 7-bit ASCII.
There's nothing we can do about that, just something to keep in mind
(it isn't supposed to happen in correct usage).