lisp/emacs-lisp/eieio*.el: Reduce object header to 1 slot

lisp/emacs-lisp/eieio*.el: Reduce object header to 1 slot

* lisp/emacs-lisp/eieio-core.el (eieio--with-scoped-class): Use let-binding.
(object): Remove first (constant) slot; rename second to `class-tag'.
(eieio--object-class-object, eieio--object-class-name): New funs
to replace eieio--object-class.
(eieio--class-object, eieio--class-p): New functions.
(same-class-fast-p): Make it a defsubst, change its implementation
to check the class objects rather than their names.
(eieio-object-p): Rewrite.
(eieio-defclass): Adjust the object initialization according to the new
object layout.
(eieio--scoped-class): Declare it returns a class object (not a class
name any more).  Adjust calls accordingly (along with calls to
eieio--with-scoped-class).
(eieio--slot-name-index): Rename from eieio-slot-name-index and change
its class arg to be a class object.  Adjust callers accordingly.
(eieio-slot-originating-class-p): Make its start-class arg a class
object.  Adjust all callers.
(eieio--initarg-to-attribute): Rename from eieio-initarg-to-attribute.
Make its `class' arg a class object.  Adjust all callers.

* lisp/emacs-lisp/eieio-base.el (eieio-persistent-validate/fix-slot-value):
Use eieio--slot-name-index rather than eieio-slot-name-index.

* lisp/emacs-lisp/eieio.el (child-of-class-p): Make it accept class objects
additionally to class names.

* test/automated/eieio-test-methodinvoke.el (eieio-test-method-store):
Adjust to new semantics of eieio--scoped-class.
(eieio-test-match): Improve error feedback.