Summary:
@public
`-ffast-math` does not have measurable performance benefits.
By using `NaN` for *undefined* values again, we can squeeze `YGFloatOptional` into 32 bits.
This will also enable us to store `YGValue` (or a variant of it) in 32 bits.
Reviewed By: astreet
Differential Revision: D13403925
fbshipit-source-id: b13d026bf556f24ab4699e65fb450af13a70961b