setf: Don't use DO to "zip up" temporaries in read-modify-write macros.
* Alter POP, REMF and DEFINE-MODIFY-MACRO to use (MAPCAR #'LIST
DUMMIES VALS) when building LET*-bindings instead of some crazy DO loop
involving PUSH and NREVERSE.
* While we're here, introduce a new temporary in POP rather than
destructively modify a binding.