Fix lexical variables and setq

[Fuco1]

See #164 and #4

(let ((my-lex-var nil))
  ;; Elsa thinks `my-lex-var' can only be nil
  (setq my-lex-var t)
  ;; Elsa thinks this conditional is always false because it thinks
  ;; the var is always nil.
  (when my-lex-var
    (message "hello"))
  my-lex-var)