Traverse subtrees with zipper and add traverse_while

[NickNeck]

Hi @doorgan

Thanks for your great work.

I wanted to propose to make it possible to traverse subtrees with Zipper.traverse.

I would also suggest the functions Zipper.traverse_while/2 and Zipper.traverse_while/3. This functions would provide :skip to skip a subtree and :halt to halt the traversing.

What do you think?

[doorgan]

I think it's a great addition. I've already had and seen use cases for traversing sub trees, and I can also see traverse_while being useful.

I merged your pull request 💜

[doorgan]

Just one use case I've seen "in the wild" is the Surface patcher, which has to get the node and rezip to traverse a subtree:
https://github.com/surface-ui/surface/blob/434e884b8e8b90e0662ca81ffa2e850ac81c07dc/lib/mix/tasks/surface/surface.init/ex_patcher_move.ex#L25-L36