FastMaths.h : Hide 'union' based casts behind a single method.
Step towards eventually using std::bit_cast. Current LHCb implementation (in LHCbMath/bit_cast.h) cannot be used cleanly due to issues with Vc.
As a side update, adds constexpr where it works OK with Vc (I thought it didn't at all, but seems OK in some situations).
Should be bit wise identical to before.