Merge: Reuse Array Iterators

on nitc/nitc/nit this reduce a lot the number of allocated iterators.

before:

* 0m7.168s
* 2,706,498 new ArrayIterator
* the second most allocated class (after NativeArray)

after:

* 0m7.060s (-1.5%)
* 785,781 new ArrayIterator (-70%)
* the 7th most allocated class

Pull-Request: #1190
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>

diff --cc lib/standard/collection/array.nit
Simple merge