Exercise 3.22: Instead of representing a queue
as a pair of pointers, we can build a queue as a procedure with local state.
The local state will consist of pointers to the beginning and the end of an
ordinary list.  Thus, the make-queue procedure will have the form


(define (make-queue)
  (let ((front-ptr … )
        (rear-ptr … ))
    ⟨definitions of internal procedures⟩
    (define (dispatch m) …)
    dispatch))

Complete the definition of make-queue and provide implementations of the
queue operations using this representation.