NAMED-CLOSURE is broken -- environment sharing

[kchanqvq]

Thanks to u/style-warning:

(defun rw (x)
  (values (named-closure:nclo reader () x)
          (named-closure:nclo writer (y) (setf x y))))

(defun rw* (x)
  (values (lambda () x)
          (lambda (y) (setf x y))))

(defun test (c)
  (multiple-value-bind (r w) (funcall c 0)
    (format t "> ~D~%" (funcall r))
    (funcall w 1)
    (format t "> ~D~%" (funcall r))))

then

CL-USER> (test #'rw)
> 0
> 0
NIL
CL-USER> (test #'rw*)
> 0
> 1
NIL

Potential fix:

1. Use reader/writer (native) closures in the funcallable instance slots.
2. If we know about the CL implementation, use the internal value cells.