Summary:
We had functions for inserting/removing a specific child, but not for simply replacing the child set with another list.
`YGNodeSetChildren()` will unhook child nodes from the parent that don't appear in the new set. We set the disconnected child node layouts to `YGLayout()` b/c that's what the rest of the code does.
`YGTraversePreOrder()` walks the tree and calls a labmda for each node. We could very easily add a post-order traversal and the ability to stop traversal if we ever want, but for now this is an MVP.
Reviewed By: Woody17
Differential Revision: D7360203
fbshipit-source-id: 32df8e1213ead03bc0a026ec4bf453bc799bb9ce
