org-(un)fill-buffer

[Psionic K]

I wrote up a small addition to the unfill package, which is very
convenient for switching hard newlines out in favor of tools like
visual-line-mode and adaptive-wrap.

The command unfilled every list and paragraph in the entire buffer.  PR is here:
https://github.com/purcell/unfill/pull/11#pullrequestreview-1812645481

Steve wants to consider it for the org package itself.  Questions from me:
1. will this be accepted?
2. where would it go?
3. because org is more general than unfill, I would instead name the
command org-fill-buffer and we can recommend that users run in a hook:
(setq-local fill-column most-positive-fixnum).  After that, every call
to `org-fill-buffer` will just do what they mean, just like how
`fill-paragraph` will respect fill-column.

I'm going to use the combination of `org-unindent-buffer` and
`org-fill-buffer` in my own personal org shortcuts, but such a command
is probably too much based on my personal taste.

I think I will be recommending this combination to users in an upcoming video:
1.  visual-fill-column
2.  visual-line-mode
3.  adaptive-wrap
4.  configuring an org mode hook for "unfill" behavior
5.  combining unindent with (un)fill in order to clean up old cruddy
hard-indented, hard-newline documents