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``
http://dlang.org/dstyle.html explicitly mentions that acronyms have same case as first letter and first letter conforms usual case rules. I don't like this but style guide is style guide.