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.
1 job for master in 90 minutes and 1 second (queued for 2 seconds)
Status Job ID Name Coverage
  Test
failed #6251
test-all

01:30:01

 
Name Stage Failure
failed
test-all Test The script exceeded the maximum execution time set for the job
  GEN      lisp/rot13-tests.log
ELC lisp/scroll-lock-tests.elc
GEN lisp/scroll-lock-tests.log
ELC lisp/ses-tests.elc
GEN lisp/ses-tests.log
ELC lisp/shadowfile-tests.elc
GEN lisp/shadowfile-tests.log
Pulling docker image gitlab/gitlab-runner-helper:x86_64-f100a208 ...
ERROR: Job failed: execution took longer than 1h30m0s seconds