bug#40693: 28.0.50; json-encode-alist changes alist

[Eli Zaretskii]

> Cc: contovob@tcd.ie, darthandrus@gmail.com, 40693@debbugs.gnu.org
> From: Dmitry Gutov <dgutov@yandex.ru>
> Date: Wed, 29 Apr 2020 17:28:05 +0300
> 
> > How about adding an optional argument instead, by default off, to
> > request this behavior?  then callers who care about the original alist
> > could request a non-destructive operation, and others won't suffer any
> > slowdown.
> 
> The current behavior is unsafe, that's the problem. Also, 
> json-encode-alist is called in a recursive fashion, so it'd have to be a 
> global variable instead.

Sounds like a somewhat hairy issue.  Then let's put this on master
first.  If this turns out to be a very popular change, and no one
complains, we could later back-port it to some 27.x version.

Thanks.