nadvice.el: Use OClosures

nadvice.el: Use OClosures

* lisp/emacs-lisp/nadvice.el (advice): New OClosure type.
(advice--how-alist): Make it hold prototype OClosures rather
than bytecode strings.
(advice--bytecodes): Delete var.
(advice--where): Make it an obsolete alias of new `advice--how`.
(oclosure-interactive-form, cl-print-object) <advice>: New methods.
(advice--make-1): Delete function.
(advice--make): Use `advice-copy` and `advice-cons`.
(advice--tweak): Use `advice-cons`.
(add-function, advice-add): Rename `where` arg to `how`.

* lisp/emacs-lisp/cl-print.el (cl-print-object) <:extra "nadvice">:
Remove now-redundant ad-hoc method.

* test/lisp/emacs-lisp/nadvice-tests.el (advice-test-print): New test.