HideShow mode

M-x hs-minor-mode

I think I rather like hs-hide-all…

(defun sacha/hs-minor-mode-hide-all ()
  "Turn on `hs-minor-mode' and hide everything."
  (hs-minor-mode 1)
  (hs-hide-all))

(add-hook 'emacs-lisp-mode-hook 'sacha/hs-minor-mode-hide-all t)