Merge: Make stream and iterators withable

As I wanted to use the `with` statement on Iterators in order to experiment with #1735, I realized that neither them not stream are usable with `with` since they lack the required methods.

Pull-Request: #1742
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>

diff --cc lib/core/stream.nit
Simple merge