php-ds: add missed return types for the getIterator() methods

[HaruAtari]

Image that you have a method which returns a generic collection:

/**
 * @return Vector<Communication>
 */
public function getCollection(): Vector{
  return new Vector([new Communication(), new Communication()]);
}

And also you have another method where you iterate over that collection:

public function test()
{
  $list = $this->getCollection();
  foreach($collection as $item){
    ..
  }
}

If both methods are in the same class, everything works good. But if these classes are in different classes, phpstorm will no understand a time of collection's item inside foreach.

Everything works properly for the Queue collection, but there is problems with other collections. I see that there is a phpdoc for the Queue::getIterator() method:

/**
 * @return Traversable<TValue>
 */
 public function getIterator(): Traversable {}

but there is no such ones for other collection.

So this MR fixes this issue