LOOP: warn when mis-destructuring with lambda list arguments

[Michał "phoe" Herda]

The LOOP macro uses its own destructuring, and not the mechanism found in destructuring lambda lists. Therefore, in (loop :for (x &rest y) := ...), LOOP will bind three variables: X, &REST, and Y.

SBCL could detect lambda list keywords passed to such variable lists and warn the programmer at compile-time that they are about to make a mistake.