Exercise 2.75. Implement the constructor make-from-mag-ang in message-passing style. This procedure should be analogous to the make-from-real-imag procedure given above.
message-passing 版本的见下：
(define (make-from-mag-ang x y)
(define (dispatch op)
(cond ((eq? op 'real-part)
(* x (cos y)))
((eq? op 'imag-part)
(* x (sin y)))
((eq? op 'magnitude) x)
((eq? op 'angle) y)
(else
(error "Unknown op -- MAKE-FROM-MAG-ANG" op))))
dispatch)