move bit_cast

Move Kernel/bit_cast.h to LHCbMath/bit_cast.h as it was already used in LHCbMath/SIMDWrapper.h, which does not depend on Kernel/LHCbKernel.

This didn't show up before because SIMDWrapper.h was not actually included by any sources files in LHCbMath (but I now want to do this...)

FYI @graven

@cattanem please unWIP/merge when convenient