1.0.17.3: unify CONSTANT nodes for DEFCONSTANT and literal constants

1.0.17.3:  unify CONSTANT nodes for DEFCONSTANT and literal constants

 * Prevents at least some cases which used to introduce duplicate
   constants into code objects.

 * The above causes all CONSTANTS being :DEFINED and anonymous, which
   is generally speaking not a problem, but causes trouble for XREF,
   which wants to generate REFERENCES information for constants as
   well. ...so, we add a %SOURCE-NAME slot to REF, allowing us to have
   references to a shared constant under multiple names.

 * Test-case.