New function flatten-tree

New function flatten-tree

Co-authored-by: Basil L. Contovounesios <contovob@tcd.ie>
* doc/lispref/lists.texi: Document `flatten-tree'.

* lisp/progmodes/js.el (js--maybe-join):
* lisp/printing.el (pr-switches):
* lisp/lpr.el (lpr-print-region):
* lisp/gnus/nnimap.el (nnimap-find-wanted-parts):
* lisp/gnus/message.el (message-talkative-question):
* lisp/gnus/gnus-sum.el (gnus-remove-thread)
(gnus-thread-highest-number, gnus-thread-latest-date):
* lisp/eshell/esh-util.el (eshell-flatten-and-stringify):
* lisp/eshell/esh-opt.el (eshell-eval-using-options):
* lisp/eshell/esh-ext.el (eshell-external-command):
* lisp/eshell/em-xtra.el (eshell/expr):
* lisp/eshell/em-unix.el (eshell/rm, eshell-mvcpln-template)
(eshell/cat, eshell/make, eshell-poor-mans-grep, eshell-grep)
(eshell/du, eshell/time, eshell/diff, eshell/locate):
* lisp/eshell/em-tramp.el (eshell/su, eshell/sudo):
* lisp/eshell/em-term.el (eshell-exec-visual):
* lisp/eshell/em-dirs.el (eshell-dirs-substitute-cd, eshell/cd):
* lisp/eshell/em-basic.el (eshell/printnl):
Use new flatten-tree.

* lisp/progmodes/js.el (js--flatten-list):
* lisp/lpr.el (lpr-flatten-list):
* lisp/gnus/message.el (message-flatten-list):
* lisp/eshell/esh-util.el (eshell-flatten-list):
Obsolete in favor of Emacs-wide `flatten-tree'.

* lisp/subr.el (flatten-list): Alias to `flatten-tree' for
discoverability.

* lisp/subr.el (flatten-tree): New defun.

* test/lisp/subr-tests.el (subr-tests-flatten-tree): New test.