Fixed two major bugs reported by Norman Werver. Unification of

Fixed two major bugs reported by Norman Werver. Unification of
strings and symbols and of numers and symbols was not recurring on
VAR-UNIFY, as required; thus

(unify '(?x ?x) '("asd" "qweert"))
and
(unify '(foo ?x baz) '(foo 42 ?x))

would succed.
The two cases are now fixed.