bug#39546: 28.0.50; Do not require subr-x at run time

[Tassilo Horn <tsdh@gnu.org>]

Eli Zaretskii <eliz@gnu.org> writes:

>> > subr.el is preloaded, so doing that will enlarge the Emacs memory
>> > footprint.  Is that function important enough to have it preloaded?
>> According with its usage at Emacs source (only at json.el) I'd say
>> the answer to your question is: not worth to be at subr.el.
>
> That's what I thought.  So I suggest to find another file to host this
> function, if we want subr-x.el to have only macros and defsubst's.

Given that it's only a wrapper around replace-buffer-contents which is
always available, I think it shouldn't be hidden too much.  AFAIK, I've
put it in subr.el first and requests were made to move it to subr-x.el.

I've already submitted patches to hindent (and I think to some other 3rd
party package providing code formatting by replacing with the output of
some formatting tool) using that function.

Anyway, if it is going to be moved, I'd to ask you to move it in
emacs-27 so that I can check for and fix external usages I know of
without having to distinguish emacs versions.

Bye,
Tassilo