Fix find-lexical-variables for LispWorks

[Yehouda]

find-lexical-variables can be made to work in LispWorks by the documented sys:map-environment:

#+lispworks (let ((list nil))
(sys:map-environment env :variable #'(lambda (name kind info)
(declare (ignore info))
(when (eq kind :lexical)
(push name list))))
(nreverse list))