Returns an iterator for the collection. (These are stateful iterators and
are not thread-safe; if you want a pure iterator, your best bet is to convert
the collection to a list.) The iterator is a closure of one argument; given
:done?, it returns true iff the iterator is exhausted; given :more?, it
returns true iff the iterator is _not_ exhausted. Given :get, if the iterator
is not exhausted, it returns the next element (or pair, for a map, as two values),
with the second value (third, for a map) being true, and advances one element; if
it is exhausted, it returns two nil values (three, for a map).