Un-deprecate oset and oset-default

Un-deprecate oset and oset-default

For discussion see the following threads:
https://lists.gnu.org/archive/html/emacs-devel/2020-05/msg00630.html
https://lists.gnu.org/archive/html/emacs-devel/2020-05/msg00674.html
https://lists.gnu.org/archive/html/emacs-devel/2020-06/msg00099.html

* lisp/emacs-lisp/eieio.el (oset, oset-default): Un-deprecate.
* lisp/emacs-lisp/eieio-core.el (eieio-oref): Declare gv-setter here
instead of in lisp/emacs-lisp/eieio.el.  Suggested by
Stefan Monnier <monnier@iro.umontreal.ca>.
(eieio-oref-default): Add gv-setter declaration.
* etc/NEWS: Announce these changes.
* doc/misc/eieio.texi (Accessing Slots): Document oref and
oref-default as generalized variables.  Consistently document
getters before setters.
* test/lisp/emacs-lisp/eieio-tests/eieio-tests.el: Use
lexical-binding.
(eieio-test-13-init-methods): Simplify.
(eieio-test-33-instance-tracker): Declare IT-list as special.