Improve replace-buffer-contents/replace-region-contents

* src/editfns.c (Freplace_buffer_contents): Add two optional arguments
  for mitigating performance issues.
* lisp/emacs-lisp/subr-x.el (replace-region-contents): Move from
  subr.el.  Add the same two arguments as for replace-buffer-contents.
* lisp/json.el (json-pretty-print-max-secs): New variable holding the
  default MAX-SECS value json-pretty-print passes to
  replace-buffer-contents.
  (json-pretty-print): Use it.
* doc/lispref/text.texi (Replacing): Add documentation for
  replace-buffer-contents two new optional arguments.  Document
  replace-region-contents.

lisp/emacs-lisp/subr-x.el

@@ -250,6 +250,35 @@ TRIM-LEFT and TRIM-RIGHT default to \"[ \\t\\n\\r]+\"."
       (substring string 0 (- (length string) (length suffix)))
     string))
 
+(defun replace-region-contents (beg end replace-fn
+                                    &optional max-secs max-costs)
+  "Replace the region between BEG and END using REPLACE-FN.
+REPLACE-FN runs on the current buffer narrowed to the region.  It
+should return either a string or a buffer replacing the region.
+
+The replacement is performed using `replace-buffer-contents'
+which also describes the MAX-SECS and MAX-COSTS arguments and the
+return value.
+
+Note: If the replacement is a string, it'll be placed in a
+temporary buffer so that `replace-buffer-contents' can operate on
+it.  Therefore, if you already have the replacement in a buffer,
+it makes no sense to convert it to a string using
+`buffer-substring' or similar."
+  (save-excursion
+    (save-restriction
+      (narrow-to-region beg end)
+      (goto-char (point-min))
+      (let ((repl (funcall replace-fn)))
+	(if (bufferp repl)
+	    (replace-buffer-contents repl max-secs max-costs)
+	  (let ((source-buffer (current-buffer)))
+	    (with-temp-buffer
+	      (insert repl)
+	      (let ((tmp-buffer (current-buffer)))
+		(set-buffer source-buffer)
+		(replace-buffer-contents tmp-buffer max-secs max-costs)))))))))
+
 (provide 'subr-x)
 
 ;;; subr-x.el ends here