Improve Growable by avoiding virtual function call.

[sundy-li]

I figured out that dyn growable runs by a slice iterator.

 let mut growable = growable::make_growable(&[lhs, rhs], false, lhs.len());
        let mut start_falsy = 0;
        let mut total_len = 0;
        for (start, len) in SlicesIterator::new(predicate.values()) 
  ...
}

what if we can put the iterator inside the growable trait?

 fn run(&mut self, iter: SlicesIterator  ) -> Box<dyn Array>;

[jorgecarleitao]

Sounds like a great idea! I would call it extend_from_iter<I: Iterator<Item=(usize, usize, usize)>> or something.