Exercise 2.30: Define a procedure
square-tree analogous to the square-list procedure of
Exercise 2.21.  That is, square-tree should behave as follows:


(square-tree
 (list 1
       (list 2 (list 3 4) 5)
       (list 6 7)))
(1 (4 (9 16) 25) (36 49))


Define square-tree both directly (i.e., without using any higher-order
procedures) and also by using map and recursion.