Modify private apis to set, store, and get intrinsic sizing keywords (#1721)

Summary:
Pull Request resolved: https://github.com/facebook/yoga/pull/1721

X-link: https://github.com/facebook/react-native/pull/46938

The private internals of how we store styles needed to change a bit to support 3 new keyword values. Right now the only other keyword that can be stored is `auto`. As a result there isn't much fancy logic to support storing this and its just stored as a specific type inside of `StyleValueHandle`. There are only 3 bits for types (8 values), so it is not sustainable to just stuff every keyword in there. So the change writes the keyword as a value with a new `keyword` `Type`.

I chose not to put `auto` in there even though it is a keyword since it is a hot path, I did not want to regress perf when I did not need to.

I also make a new `StyleSizeValue` class to store size values - so values for `width`, `height`, etc. This way these new keywords are kept specific to sizes and we will not be able to create, for example, a margin: `max-content`.

Changelog: [Internal]

Reviewed By: NickGerleman

Differential Revision: D63927512

fbshipit-source-id: 7285469d37ac4b05226183b56275c77f0c06996c

yoga/YGValue.h

@@ -65,6 +65,9 @@ inline bool operator==(const YGValue& lhs, const YGValue& rhs) {
   switch (lhs.unit) {
     case YGUnitUndefined:
     case YGUnitAuto:
+    case YGUnitFitContent:
+    case YGUnitMaxContent:
+    case YGUnitStretch:
       return true;
     case YGUnitPoint:
     case YGUnitPercent: