Exercise 2.28: Write a procedure fringe
that takes as argument a tree (represented as a list) and returns a list whose
elements are all the leaves of the tree arranged in left-to-right order.  For
example,


(define x 
  (list (list 1 2) (list 3 4)))

(fringe x)
(1 2 3 4)

(fringe (list x x))
(1 2 3 4 1 2 3 4)