bug#33309: Add flatten-list?

[Drew Adams <drew.adams@oracle.com>]

FWIW, I don't think Emacs Lisp needs a flatten 
function.

1. Even Common Lisp doesn't bother with one.
   (And there are lots of Common Lisp
   functions I'd sooner see added to Emacs.)

2. It's trivial to define when needed, and
   there are not a lot of existing uses of it.

3. The functions you mention do not all behave
   the same.  Even their arg lists are not the 
   same.  (But sure, they could all be made to
   use a common version, if that were important.)

4. (flatten-list 42) => (42)

   The doc string says nothing about the case
   where the argument LIST (a bad name here)
   is not a list.  And if the arg need not be
   a list then "-list" in the function name
   is not good.