Add f16, f128

[usamoi]

1. integer_decode is implemented for f128 by truncating the radix to its 64 most significant bits. See Float::integer_decode cannot be implemented for quadruple precision floating types. #98.
2. Because of precision limitations, f128 tests under no_std and libm fail.

[cuviper]

I'm not keen on maintaining for unstable features in this crate, so we're probably going to be blocked here until the language and standard library stabilize these new floats.

At that point, I think we'll probably want to have detection in the build script to enable it automatically, and Cargo features can force that on -- like we used to for has_i128.

[SciMind2460]

rust-lang/libs-team#707 would eventually rely on this (as per my implementation) so could we expedite support?

[cuviper]

rust-lang/libs-team#707 would eventually rely on this (as per my implementation) so could we expedite support?

I would not expect the standard library to start using num-traits, especially because that new Complex type should probably be in core, and you can't build num-traits as a dependency before that! I would expect a pseudo-internal trait instead, more like the setup of struct NonZero<T> where T: ZeroablePrimitive.

(My reservations remain about unstable features as well.)