SETF of PROGN

[Guy.Steele at CMU-10A]

While we are fixing bugs in SETF why not add the setf-x properties for
fixnum and flonum-identity?

And while you are at it you might also want to do PROGN:

(setf (progn x) 12) = (setf x 12)

[More generally,
	(setf (progn x1 x2 ... xn) 12) = (progn x1 x2 ... (setf xn 12))
-- Guy]