Exercise 1.18.  Using the results of exercises 1.16 and 1.17, devise a 
procedure that generates an iterative process for multiplying two integers in 
terms of adding, doubling, and halving and uses a logarithmic number of steps.

(define (* a b)
  (times-iter a b 0))

(define (times-iter a b n)
  (cond ((= b 0) n)
        ((even? b) (times-iter (double a) (halve b) n))
        (else (times-iter a (- b 1) (+ a n)))))