bug#70007: [PATCH] native JSON encoder

[Eli Zaretskii <eliz@gnu.org>]

> From: Mattias Engdegård <mattias.engdegard@gmail.com>
> Date: Tue, 26 Mar 2024 16:33:52 +0100
> 
> If we replace the lisp-to-JSON encoder with native code, we would not need the jansson library for it and it would be faster.
> 
> There is ongoing work on a JSON-to-lisp parser, but the author has made it clear that he does not have time to write an encoder, so I spent a morning mashing up the attached patch.

Thanks for working on this.

> It generally produces the same result as the old code, except:
> 
> - The old code incorrectly accepted strings with non-Unicode characters (raw bytes). There is no reason to do this; JSON is UTF-8 only.

Would it complicate the code not to reject raw bytes?  I'd like to
avoid incompatibilities if it's practical.  Also, Emacs traditionally
doesn't reject raw bytes, leaving that to the application or the user.

> I'd be very happy if someone could test it with packages that use this interface (json-serialise, json-insert).

Yes, please.