Move out YGValue

Summary:
@public

Creates a single header file for `YGValue`. This is in preparation of a more compact representation of `YGValue` within `YGStyle`.

Also fixes the incorrect definition of NAN.

Reviewed By: SidharthGuglani

Differential Revision: D13172444

fbshipit-source-id: 4250dbcf8fe15ec3ecdee3913360a73bab633ce3

yoga/Yoga.cpp

@@ -39,23 +39,6 @@ static int YGDefaultLog(
     va_list args);
 #endif
 
-const YGValue YGValueZero = {0, YGUnitPoint};
-const YGValue YGValueUndefined = {YGUndefined, YGUnitUndefined};
-const YGValue YGValueAuto = {YGUndefined, YGUnitAuto};
-
-bool operator==(const YGValue& lhs, const YGValue& rhs) {
-  if ((lhs.unit == YGUnitUndefined && rhs.unit == YGUnitUndefined) ||
-      (lhs.unit == YGUnitAuto && rhs.unit == YGUnitAuto)) {
-    return true;
-  }
-
-  return lhs.unit == rhs.unit && lhs.value == rhs.value;
-}
-
-bool operator!=(const YGValue& lhs, const YGValue& rhs) {
-  return !(lhs == rhs);
-}
-
 #ifdef ANDROID
 #include <android/log.h>
 static int YGAndroidLog(