(outline--hidden-headings-paths): Fix slow saves (bug#78665)

(outline--hidden-headings-paths): Fix slow saves (bug#78665)

* lisp/outline.el: Prefer #' to quote function names.
(outline--end-of-previous): New function, extracted from
`outline-end-of-subtree`.
(outline-end-of-subtree): Use it.
(outline--hidden-headings-paths): Distinguish headings where just the
entry is hidden from those where a whole subtree is hidden (bug#78673).
(outline--hidden-headings-restore-paths): Adjust accordingly and don't
delegate to functions like `outline-hide-subtree` so as to avoid
an O(N²) behavior.

(cherry picked from commit 77a4c63fda5ca5d4c6d82092eaa06f1eb9b51302)