Return values of SETF

[Daniel L. Weinreb <dlw at MIT-AI>]

I'm pretty much convinced by Masinter's mail.  SETF should be defined to
return the value that it stores.  SETF is really too important a form to
work in an explicitly undefined method, and compiler optimizations
and/or special-purpose settting functions (that exist only so that SETF
can turn into them) are well worth it to keep SETF from having to have
crummy "undefined" behavior.  (Not having any kind of up-to-date Common
Lisp manual, I have no idea how or if it is currently defined.)