ANSI-compliant DEFSETF.

[davidmullen]

Resolves #38. This implementation probably needs more testing, but at least it does pass the ANSI test suite.

As a bonus, this allows the use of &environment in defsetf, which most Common Lisp implementations don't:

• CCL doesn't: Defsetf Lambda-Lists Don't Permit the &environment Parameter
• ECL doesn't either: &environment in setf-lambda-lists leads to call failure

Refer to ANSI: 3.4.7 Defsetf Lambda Lists.