Bug fix: list? now O(log n)

[David Van Horn <dvanhorn@xxxxxxxxxxxxxxx>]

I noticed a really dumb bug in the reference implementation for list?, 
which the document requires to be O(log n), where n is the count of the 
chain of pairs in the input.  Instead the implementation was naively 
traversing through the list of elements instead of just the forest of 
trees.  This will be fixed in the next revision, but the change is the 
following:

  ;; [Any -> Boolean]
  ;; Is x a PROPER list?
  (define (ra:list? x)
    (or (ra:null? x)
        (and (ra:pair? x)
             (ra:list? (ra:cdr x)))))

To:

  ;; [Any -> Boolean]
  ;; Is x a PROPER list?
  (define (ra:list? x)
    (or (ra:null? x)
        (and (kons? x)
             (ra:list? (kons-rest x)))))

David