(replace-region-contents): Improve and promote (bug#76313)

Swap the role of `replace-region-contents` and `replace-buffer-contents`,
so `replace-region-contents` is the main function, implemented in C,
and `replace-buffer-contents` is a mere wrapper (marked as obsolete).
Also remove the need to rely on narrowing and on describing the
new text as a function.
Finally, allow MAX-SECS==0 to require a cheap replacement, and
add an INHERIT argument.

* src/editfns.c: Include `coding.h`.
(Freplace_region_contents): Rename from `Freplace_buffer_contents`.
Change calling convention to that of `replace-region-contents`.
Add more options for the SOURCE argument.  Add INHERIT argument.
Skip the costly algorithm if MAX-SECS is 0.
* src/insdel.c (replace_range): Allow NEW to be a buffer.

* lisp/subr.el (replace-buffer-contents): New implementation.
* lisp/emacs-lisp/subr-x.el (replace-region-contents): Delete.

* doc/lispref/text.texi (Replacing): Document new API for
`replace-region-contents`.  Remove documentation of
`replace-buffer-contents`.

* test/src/editfns-tests.el (replace-buffer-contents-1)
(replace-buffer-contents-2, replace-buffer-contents-bug31837):
Use `replace-region-contents`.
(editfns--replace-region): Delete.
(editfns-tests--replace-region): Use `replace-region-contents`.
Adds tests for new types of SOURCE args.

src/coding.c

@@ -7898,6 +7898,8 @@ code_conversion_save (bool with_work_buf, bool multibyte)
       bset_enable_multibyte_characters (current_buffer, multibyte ? Qt : Qnil);
       if (EQ (workbuf, Vcode_conversion_reused_workbuf))
 	reused_workbuf_in_use = true;
+      /* FIXME: Maybe we should stay in the new workbuf, because we often
+	 switch right back to it anyway in order to initialize it further.  */
       set_buffer_internal (current);
     }