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/algorithm/Baseline.cpp

@@ -34,8 +34,8 @@ float calculateBaseline(const yoga::Node* node, void* layoutContext) {
   }
 
   yoga::Node* baselineChild = nullptr;
-  const uint32_t childCount = YGNodeGetChildCount(node);
-  for (uint32_t i = 0; i < childCount; i++) {
+  const size_t childCount = node->getChildCount();
+  for (size_t i = 0; i < childCount; i++) {
     auto child = node->getChild(i);
     if (child->getLineIndex() > 0) {
       break;