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.
Edited by Marco Cattaneo