Fix earlier half-done eieio-defmethod change.

Fix earlier half-done eieio-defmethod change.
* lisp/emacs-lisp/eieio.el (eieio--defmethod): Rename from eieio-defmethod.
Streamline and change calling convention.
(defmethod): Adjust accordingly and simplify.
(eieio-defclass): Fix broken calls to eieio-defmethod and redirect to
new eieio--defmethod.
(slot-boundp): Minor CSE simplification.

Fixes: debbugs:8338