1.0.17.25: allow dumping of references to arbitrary named constants

1.0.17.25: allow dumping of references to arbitrary named constants

 * While ANSI does not require us to do this, supporting this allows
   users to write code like:

     (unless (boundp 'f) (defconstant f (lambda () 'foo!)))
     (defun foo () f)

   ...which pre 1.0.17.3 SBCL also allowed.