Add float types with niche

[scottmcm]

@kpreid

and which can provide niches, Eq, and Ord, does seem worthwhile.

Yes, and conveniently things like infinities are not a problem for Ord:

Suggestion: FFastMath intrinsics in stable

I would propose NonNanF32 (see threads like [Pre-RFC?] NonNaN type), since ±∞ really don't cause the same problems that NANs do. (Including the infinities is still Eq+Ord, for example.) And things like log(0) => -∞ and exp(-∞) => 0 are quite handy.

Really, for computation probably the right path will basically always be to turn them back into f32s, do lots of computation, and then only at the end convert back. Checking in rust for NAN at every step is probably going to destroy the perf.