Implement IntoIterator for IMap and IArray via their refs

[kirillsemyonkin]

This PR is a variant of implementing IntoIterator via an &'a reference to the respective collections (IArray, IMap).

This means the iterator may not outlive the instance. For example, you may not return the iterator from a function, if the supplied IArray does not outlive the function scope:

fn first<'a>(arr: &'a IArray<i32>) -> impl Iterator<Item = i32> + 'a { // note that implemented object lives as long as that reference - it borrows `arr`
    arr.into_iter() // ok in any solution
}

fn second() -> impl Iterator<Item = i32> {
    let arr = IArray::...;
    arr.into_iter() // cannot happen, as `IArray` will be `Drop`ped at the end of this scope
}

The IArray implementation is rewritten to the same approach for consistency. Breaking change.

Closes #69 because the two approaches are disjoint.