Use bitfields for YGLayout and YGNode

Summary:
@public

Further heap size reductions by using bitfields in `YGLayout` and `YGNode`.

Reviewed By: SidharthGuglani

Differential Revision: D13466325

fbshipit-source-id: ddcef0a1b3822e7449fe485d99c920d54139c893

yoga/YGLayout.h

@@ -17,11 +17,13 @@ struct YGLayout {
   std::array<float, 6> margin = {};
   std::array<float, 6> border = {};
   std::array<float, 6> padding = {};
-  YGDirection direction = YGDirectionInherit;
+  YGDirection direction : 2;
+  bool didUseLegacyFlag : 1;
+  bool doesLegacyStretchFlagAffectsLayout : 1;
+  bool hadOverflow : 1;
 
   uint32_t computedFlexBasisGeneration = 0;
   YGFloatOptional computedFlexBasis = {};
-  bool hadOverflow = false;
 
   // Instead of recomputing the entire layout every single time, we
   // cache some information to break early when nothing changed
@@ -34,10 +36,12 @@ struct YGLayout {
   std::array<float, 2> measuredDimensions = kYGDefaultDimensionValues;
 
   YGCachedMeasurement cachedLayout = YGCachedMeasurement();
-  bool didUseLegacyFlag = false;
-  bool doesLegacyStretchFlagAffectsLayout = false;
 
-  YGLayout() = default;
+  YGLayout()
+      : direction(YGDirectionInherit),
+        didUseLegacyFlag(false),
+        doesLegacyStretchFlagAffectsLayout(false),
+        hadOverflow(false) {}
 
   bool operator==(YGLayout layout) const;
   bool operator!=(YGLayout layout) const {