Introduce CompactValue

Summary:
@public

`CompactValue` represents a `YGValue` in 32bits instead of 64. This comes at the cost of a range limitation, as one exponent bit is borrowed for the unit.

*Undefined* and *Auto* have no magnitude, and are represented as *NaN* values.

The data structure is meant to be used as a field type on `YGStyle` to save memory.

This is header-only for efficient inlining.

Reviewed By: jackerghan, aCorrado

Differential Revision: D13187211

fbshipit-source-id: 16e3ffad592e38e2493e4f7c8b952d372e449846

BUCK

@@ -20,6 +20,7 @@ TEST_COMPILER_FLAGS = BASE_COMPILER_FLAGS + GMOCK_OVERRIDE_FLAGS + [
 yoga_cxx_library(
     name = "yoga",
     srcs = glob(["yoga/*.cpp"]),
+    headers = subdir_glob([("", "yoga/**/*.h")]),
     header_namespace = "",
     exported_headers = subdir_glob([("", "yoga/*.h")]),
     compiler_flags = COMPILER_FLAGS,
@@ -34,6 +35,7 @@ yoga_cxx_library(
 yoga_cxx_test(
     name = "YogaTests",
     srcs = glob(["tests/*.cpp"]),
+    headers = subdir_glob([("", "yoga/**/*.h")]),
     compiler_flags = TEST_COMPILER_FLAGS,
     contacts = ["emilsj@fb.com"],
     visibility = ["PUBLIC"],