1.0.3.5: slightly different SEQUENCE type handling.

1.0.3.5: slightly different SEQUENCE type handling.
	The UNION TYPE= issue affects type derivation as in
	(defun foo (x)
          (declare (type (simple-array character) x))
          (subseq x 1 2))
        where the system fails to derive that the intersection of
	consed-sequence and (simple-array character) is distinct from
	consed-sequence.  Change SEQUENCE to be an explicit union of
	LIST, VECTOR and an EXTENDED-SEQUENCE named type, defining
	appropriate type methods, and the symptom (but not the cause)
	goes away.

	(Note: it may well be that the EXTENDED-SEQUENCE named type
	disappears again, to be replaced by an actual protocol class
	similar to FUNDAMENTAL-STREAM for Gray streams, for reasons of
	future extensibility and ease of compatibility with other Lisps.
        Waiting for ILC feedback...)