Add f16, f128

[usamoi]

1. integer_decode is implemented for f128 by truncating the radix to its 64 most significant bits. See #98.
2. Because of precision limitations, f128 tests under no_std and libm fail.

[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.)