Exercise 4.7: Let* is similar to
let, except that the bindings of the let* variables are performed
sequentially from left to right, and each binding is made in an environment in
which all of the preceding bindings are visible.  For example


(let* ((x 3)
       (y (+ x 2))
       (z (+ x y 5)))
  (* x z))

returns 39.  Explain how a let* expression can be rewritten as a set of
nested let expressions, and write a procedure let*->nested-lets
that performs this transformation.  If we have already implemented let
(Exercise 4.6) and we want to extend the evaluator to handle let*,
is it sufficient to add a clause to eval whose action is


(eval (let*->nested-lets exp) env)

or must we explicitly expand let* in terms of non-derived expressions?