This PR adds an iterator adapter to make hardware usage counting easier when we are measuring iterators.

This adapter plugs in easily to an iterator. When the iterator gets dropped, it will execute the callback to do something with the final number of iterations the iterator was used for.

For example:

index.value_to_points.get_iter(value).on_final_count(|nexts| {
    hw_counter
        .payload_index_io_read_counter()
        .incr_delta(nexts * size_of::<PointOffsetType>)
});

I am not sure about the name, though. Feel free to propose another one :)