Add something like `num_traits` to STD, or move it into std entirely

SciMind2460:

But there isn't a plan for generic floating-point numbers that I know of

Now that we have 4 floating point types in the works, we could make f<N> work if we wanted -- probably with a trait bound of some sort since I doubt we'd even enable things like f<7> or f<123> the way that would be fine for integers.