New comment-sexp command and key bind

[Artur Malabarba <bruce.connor.am@gmail.com>]

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I'd like to add the following command that comments and uncomments
sexps. I'm attacking a gif to show its behavior, but it is pretty much
the same as our current comment-line, except it acts on sexps.

Furthermore, I'd like to bind it to `C-M-;' since C-M- is a relatively
common modifier for sexp-stuff.

-----
(defun uncomment-sexp (&optional n)
  "Uncomment a sexp around point."
  (interactive "P")
  (let* ((initial-point (point-marker))
         (p)
         (end (save-excursion
                (when (elt (syntax-ppss) 4)
                  (re-search-backward comment-start-skip
                                      (line-beginning-position)
                                      t))
                (setq p (point-marker))
                (comment-forward (point-max))
                (point-marker)))
         (beg (save-excursion
                (forward-line 0)
                (while (= end (save-excursion
                                (comment-forward (point-max))
                                (point)))
                  (forward-line -1))
                (goto-char (line-end-position))
                (re-search-backward comment-start-skip
                                    (line-beginning-position)
                                    t)
                (while (looking-at-p comment-start-skip)
                  (forward-char -1))
                (point-marker))))
    (unless (= beg end)
      (uncomment-region beg end)
      (goto-char p)
      ;; Indentify the "top-level" sexp inside the comment.
      (while (and (ignore-errors (backward-up-list) t)
                  (>= (point) beg))
        (skip-chars-backward (rx (syntax expression-prefix)))
        (setq p (point-marker)))
      ;; Re-comment everything before it.
      (ignore-errors
        (comment-region beg p))
      ;; And everything after it.
      (goto-char p)
      (forward-sexp (or n 1))
      (skip-chars-forward "\r\n[:blank:]")
      (if (< (point) end)
          (ignore-errors
            (comment-region (point) end))
        ;; If this is a closing delimiter, pull it up.
        (goto-char end)
        (skip-chars-forward "\r\n[:blank:]")
        (when (= 5 (car (syntax-after (point))))
          (delete-indentation))))
    ;; Without a prefix, it's more useful to leave point where
    ;; it was.
    (unless n
      (goto-char initial-point))))

(defun comment-sexp--raw ()
  "Comment the sexp at point or ahead of point."
  (pcase (or (bounds-of-thing-at-point 'sexp)
             (save-excursion
               (skip-chars-forward "\r\n[:blank:]")
               (bounds-of-thing-at-point 'sexp)))
    (`(,l . ,r)
     (goto-char r)
     (skip-chars-forward "\r\n[:blank:]")
     (comment-region l r)
     (skip-chars-forward "\r\n[:blank:]"))))

(defun comment-or-uncomment-sexp (&optional n)
  "Comment the sexp at point and move past it.
If already inside (or before) a comment, uncomment instead.
With a prefix argument N, (un)comment that many sexps."
  (interactive "P")
  (if (or (elt (syntax-ppss) 4)
          (< (save-excursion
               (skip-chars-forward "\r\n[:blank:]")
               (point))
             (save-excursion
               (comment-forward 1)
               (point))))
      (uncomment-sexp n)
    (dotimes (_ (or n 1))
      (comment-sexp--raw))))

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