Returns #t if an iterator has no more items to produce.

For a finished iterator, iter-next! will always return #n. However, returning #n does not imply that the iterator is necessarily finished.

(let it (iter '(0 #n)))
(prn (iter-next! it))     ; prints 0
(prn (iter-finished? #t)) ; prints #f
(prn (iter-next! it))     ; prints #n
(prn (iter-finished? #t)) ; prints #f
(prn (iter-next! it))     ; prints #n
(prn (iter-finished? #t)) ; prints #t

iter-finished? is updated after calling iter-next!. It won't return #t until iter-next! has returned #n at least once.