(hs-hide-comments-when-hiding-all): Remove autoload cookie.

(hs-hide-comments-when-hiding-all): Remove autoload cookie.
(hs-allow-nesting): New user var.
(hs-discard-overlays): Skip "internal" overlays if nesting allowed.
(hs-hide-block-at-point): When nesting allowed,
if there is already an overlay in place, delete it.
(hs-safety-is-job-n): Delete func; remove call sites.
(hs-hide-level-recursive): Don't pre-clean if nesting allowed.
(hs-overlay-at): New func.
(hs-already-hidden-p, hs-show-block): Use it.
(hs-hide-all): Don't pre-clean if nesting allowed.
(hs-show-all): Temporarily disallow
nesting around call to `hs-discard-overlays'.