Exercise 2.21: The procedure square-list
takes a list of numbers as argument and returns a list of the squares of those
numbers.


(square-list (list 1 2 3 4))
(1 4 9 16)


Here are two different definitions of square-list.  Complete both of
them by filling in the missing expressions:


(define (square-list items)
  (if (null? items)
      nil
      (cons ⟨??⟩ ⟨??⟩)))

(define (square-list items)
  (map ⟨??⟩ ⟨??⟩))