more conservative bounds in FP interval arithmetic

more conservative bounds in FP interval arithmetic

 Make BOUND-BINOP return closed intervals when floating
 point rounding can cause an open bound to close.

 Consider:

  ;; Can return zero.
  (defun fii (x)
    (declare (type (single-float (0.0)) x))
    (/ x 2.0))

  ;; Can return 2.0.
  (defun fii (x y)
    (declare (type (single-float 2.0) x)
             (type (single-float (0.0)) y))
    (+ x y))