Fix slot typecheck in eieio-persistent

Fix slot typecheck in eieio-persistent

* lisp/emacs-lisp/eieio-base.el (eieio-persistent-slot-type-is-class-p):
  An `or' form can specify multiple potential classes (or null) as
  valid types for a slot, but previously only the final element of the
  `or' was actually checked. Now returns all valid classes in the `or'
  form.
  (eieio-persistent-validate/fix-slot-value): Check if proposed value
  matches any of the valid classes.
* test/lisp/emacs-lisp/eieio-tests/eieio-test-persist.el
  (eieio-test-multiple-class-slot): Test this behavior.