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``
I'm OK with `IEEEHalf` and `IBMExtended`, because `IEEE` and `IBM` *are* acronyms after all, and `ieee` reads like screaming. Just my $0.02.