Add enum RealFormat to std.math

As discussed in https://github.com/D-Programming-Language/phobos/pull/2305

Replaces cryptic checks of ``T.mant_dig == 24`` with ``floatTraits!(T).realFormat == RealFormat.IEEESingle``
Fair enough. It *is* just bikeshedding at this point, let's just change it to `ieeeXXX` and move on to more important things. :-)