Use bitfields for enum members of YGStyle

Summary:
@public

Puts all enum fields of `YGStyle` into bitfields. This saves 36 bytes (> 3%) per node.

Reviewed By: SidharthGuglani

Differential Revision: D13233686

fbshipit-source-id: 3ef7e0d6913f0254806acb942d9a9f5b78a5af9c

yoga/YGStyle.h

@@ -30,16 +30,16 @@ constexpr std::array<YGValue, 2> kYGDefaultDimensionValuesUnit = {
 struct YGStyle {
   using Dimensions = std::array<YGValue, 2>;
 
-  YGDirection direction = YGDirectionInherit;
-  YGFlexDirection flexDirection = YGFlexDirectionColumn;
-  YGJustify justifyContent = YGJustifyFlexStart;
-  YGAlign alignContent = YGAlignFlexStart;
-  YGAlign alignItems = YGAlignStretch;
-  YGAlign alignSelf = YGAlignAuto;
-  YGPositionType positionType = YGPositionTypeRelative;
-  YGWrap flexWrap = YGWrapNoWrap;
-  YGOverflow overflow = YGOverflowVisible;
-  YGDisplay display = YGDisplayFlex;
+  YGDirection direction : 2;
+  YGFlexDirection flexDirection : 2;
+  YGJustify justifyContent : 3;
+  YGAlign alignContent : 3;
+  YGAlign alignItems : 3;
+  YGAlign alignSelf : 3;
+  YGPositionType positionType : 1;
+  YGWrap flexWrap : 2;
+  YGOverflow overflow : 2;
+  YGDisplay display : 1;
   YGFloatOptional flex = {};
   YGFloatOptional flexGrow = {};
   YGFloatOptional flexShrink = {};
@@ -54,7 +54,17 @@ struct YGStyle {
   // Yoga specific properties, not compatible with flexbox specification
   YGFloatOptional aspectRatio = {};
 
-  YGStyle() = default;
+  YGStyle()
+      : direction(YGDirectionInherit),
+        flexDirection(YGFlexDirectionColumn),
+        justifyContent(YGJustifyFlexStart),
+        alignContent(YGAlignFlexStart),
+        alignItems(YGAlignStretch),
+        alignSelf(YGAlignAuto),
+        positionType(YGPositionTypeRelative),
+        flexWrap(YGWrapNoWrap),
+        overflow(YGOverflowVisible),
+        display(YGDisplayFlex) {}
   bool operator==(const YGStyle& style);
 
   bool operator!=(YGStyle style) {