Exercise 4.8: “Named let” is a variant
of let that has the form 


(let ⟨var⟩ ⟨bindings⟩ ⟨body⟩)

The ⟨bindings⟩ and ⟨body⟩ are just as in ordinary let,
except that ⟨var⟩ is bound within ⟨body⟩ to a procedure whose body
is ⟨body⟩ and whose parameters are the variables in the ⟨bindings⟩.
Thus, one can repeatedly execute the ⟨body⟩ by invoking the procedure
named ⟨var⟩.  For example, the iterative Fibonacci procedure 
(1.2.2) can be rewritten using named let as follows:


(define (fib n)
  (let fib-iter ((a 1) (b 0) (count n))
    (if (= count 0)
        b
        (fib-iter (+ a b) 
                  a 
                  (- count 1)))))

Modify let->combination of Exercise 4.6 to also support named
let.