Make Iterator covariant

[VincentLanglet]

Q	A
Type	improvement
BC Break	no
Fixed issues

Summary

Since \Iterator is covariant, the mongo Iterator could be too.

This allows to write

/**
 * @template-covariant T of object
 */
interface MyCustomClass 
{
     /** @return Iterator<T> */
     public function getIrator(): Iterator
}

or to pass an Iterator to a method which works on Iterator

[malarzm]

@VincentLanglet we've managed to get checks back to shape. May I ask you to rebase your PR atop newly created 2.5.x branch?

[VincentLanglet]

@VincentLanglet we've managed to get checks back to shape. May I ask you to rebase your PR atop newly created 2.5.x branch?

Sure, done

[malarzm]

Thanks @VincentLanglet!