Breaking: size_t indices (#1366)

Summary:
Pull Request resolved: https://github.com/facebook/yoga/pull/1366

X-link: https://github.com/facebook/react-native/pull/39371

Yoga's public API exposes indices most often as `uint32_t`, with exception of clone callbacks which are `int32_t`. Yoga internally represents these indices as `size_t` when dealing with the child vector, and this is the true index.

This changes the API to consistently be `size_t`. This should not be breaking for most users, but will cause breaks where:

1. Users set a clone node callback (I think this should be rare. RN uses it, but only because it relies on a separate private API).
2. Callers of `YGNodeGetChildCount()` are assigning to an int with less width than `size_t` and have strong warnings enabled.
3. Using a newer Yoga binary with older source, since we are not preserving ABI compatibility (Yoga in general does not aim to be ABI stable between major versions, only ABI safe for a given set of sources).

Changelog: [Internal]

Reviewed By: sammy-SC

Differential Revision: D49130914

fbshipit-source-id: 6a004c160c4c50f68047b108508fd437156f5fac

yoga/node/Node.cpp

@@ -484,7 +484,7 @@ YOGA_EXPORT void Node::clearChildren() {
 // Other Methods
 
 void Node::cloneChildrenIfNeeded(void* cloneContext) {
-  int i = 0;
+  size_t i = 0;
   for (Node*& child : children_) {
     if (child->getOwner() != this) {
       child = resolveRef(config_->cloneNode(child, this, i, cloneContext));