Improve paragraph filling for nested comments.

Change paragraph-start and paragraph-separate so that haddock (-- |)
and nested comments become paragraphs; only empty lines used to be
paragraph separators. This also makes forward/backward-paragraph more
helpful.

Change haskell-fill-paragraph to recognize nested comments slightly
more accurately using forward-sexp. Also, if fill-paragraph was called
in a nested and no actual filling is necessary, make sure
non-comment-aware filling does not happen instead.