Change the eieio-declare-slots function into a macro

* lisp/emacs-lisp/eieio-core.el (eieio-declare-slots): Change into
a compile-only macro.

lisp/emacs-lisp/eieio-core.el

@@ -1085,9 +1085,10 @@ method invocation orders of the involved classes."
 These match if the argument is the name of a subclass of CLASS."
   (list eieio--generic-subclass-generalizer))
 
-(defun eieio-declare-slots (&rest slots)
+(defmacro eieio-declare-slots (&rest slots)
   "Declare that SLOTS are known eieio object slot names."
-  (setq eieio--known-slot-names (append slots eieio--known-slot-names)))
+  `(eval-when-compile
+     (setq eieio--known-slot-names (append ',slots eieio--known-slot-names))))
 
 (provide 'eieio-core)