Add YGNodeSetChildren(), YGNodeTraversePreOrder()

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

yoga/Yoga.h

@@ -84,6 +84,7 @@ WIN_EXPORT void YGNodeRemoveAllChildren(const YGNodeRef node);
 WIN_EXPORT YGNodeRef YGNodeGetChild(const YGNodeRef node, const uint32_t index);
 WIN_EXPORT YGNodeRef YGNodeGetParent(const YGNodeRef node);
 WIN_EXPORT uint32_t YGNodeGetChildCount(const YGNodeRef node);
+WIN_EXPORT void YGNodeSetChildren(YGNodeRef const parent, const YGNodeRef children[], const uint32_t count);
 
 WIN_EXPORT void YGNodeCalculateLayout(const YGNodeRef node,
                                       const float availableWidth,
@@ -298,3 +299,15 @@ WIN_EXPORT float YGRoundValueToPixelGrid(
     const bool forceFloor);
 
 YG_EXTERN_C_END
+
+#ifdef __cplusplus
+
+#include <functional>
+#include <vector>
+
+// Calls f on each node in the tree including the given node argument.
+extern void YGTraversePreOrder(YGNodeRef const node, std::function<void(YGNodeRef node)>&& f);
+
+extern void YGNodeSetChildren(YGNodeRef const parent, const std::vector<YGNodeRef> &children);
+
+#endif