Call measure and baseline fns within YGNode

Summary:
@public

Stricter encapsulation of baseline and measure callbacks withing `YGNode`.

Instead of invoking these callbacks directly (`node->getBaseline()(...)`), they are invoked via methods on `YGNode` (`node->baseline(...)`).

This change will allow us to add the concept of a *Layout Context,* where measure and baseline functions will be able to receive an additional `void *` argument if configured accordingly. This API will be used internally for Yoga’s JNI bindings, to avoid storing a weak JNI reference for each node, and avoid reference table overflows.

Changed API:

- `YGNodeGetMeasureFunc()` -> `YGNodeHasMeasureFunc()`
- `YGNodeGetBaselineFunc()` -> `YGNodeHasBaselineFunc()`
- `YGNode::getMeasure()` -> `YGNode::hasMeasureFunc()` + `YGNode::measure()`
- `YGNpde::getBaseline()` -> `YGNode::hasBaselineFunc()` + `YGNode::baseline()`

Reviewed By: SidharthGuglani

Differential Revision: D14099550

fbshipit-source-id: 2653ab36acc252a9747986bc88d21dac22d8c91b

YogaKit/Source/YGLayout.m

@@ -180,7 +180,7 @@ static YGConfigRef globalConfig;
   // the measure function. Since we already know that this is a leaf,
   // this *should* be fine. Forgive me Hack Gods.
   const YGNodeRef node = self.node;
-  if (YGNodeGetMeasureFunc(node) == NULL) {
+  if (YGNodeHasMeasureFunc(node)) {
     YGNodeSetMeasureFunc(node, YGMeasureView);
   }