Implement gap/row-gap/column-gap (within the C ABI)

Summary:
This extracts the core changes from https://github.com/facebook/yoga/pull/1116, to support gap/row-gap/column-gap, mostly identical, apart from the rename of gaps -> gutters.

The core functionality in this PR looks to be well tested from the fixtures added. I am not an expert in the internals of Yoga, but I am seeing everything that I would expect to. The space for the gap is accounted for in line-breaking, and the accumulated gaps limit the available line-length, before sizing flexible children, so items are sized correctly as to accommodate the gap. Then the gap is used for spacing during main axis and cross-axis justification.

Changelog:
[Genral][Added] - Implement gap/row-gap/column-gap (within the yoga C ABI)

Reviewed By: javache

Differential Revision: D39922410

fbshipit-source-id: 5850f22032169028bd8383b49dd240b335c11d3d

yoga/YGNode.h

@@ -204,6 +204,14 @@ public:
       YGEdge edge,
       CompactValue defaultValue);
 
+  static CompactValue computeRowGap(
+      const YGStyle::Gutters& gutters,
+      CompactValue defaultValue);
+
+  static CompactValue computeColumnGap(
+      const YGStyle::Gutters& gutters,
+      CompactValue defaultValue);
+
   // Methods related to positions, margin, padding and border
   YGFloatOptional getLeadingPosition(
       const YGFlexDirection axis,
@@ -236,6 +244,9 @@ public:
   YGFloatOptional getMarginForAxis(
       const YGFlexDirection axis,
       const float widthSize) const;
+  YGFloatOptional getGapForAxis(
+      const YGFlexDirection axis,
+      const float widthSize) const;
   // Setters
 
   void setContext(void* context) { context_ = context; }