slot-definitions must retain the generic functions of accessors

[Nikodemus Siivola]

  reported by Tony Martinez:
    (defclass foo () ((bar :reader foo-bar)))
    (defun foo-bar (x) x)
    (defclass foo () ((bar :reader get-bar))) ; => error, should work

  Note: just punting the accessor removal if the fdefinition
  is not a generic function is not enough:

   (defclass foo () ((bar :reader foo-bar)))
   (defvar *reader* #'foo-bar)
   (defun foo-bar (x) x)
   (defclass foo () ((bar :initform 'ok :reader get-bar)))
   (funcall *reader* (make-instance 'foo)) ; should be an error, since
                                           ; the method must be removed
                                           ; by the class redefinition

See also bug #309066.