Implement nightly implementation for Ord

[daxpedda]

This is related to #5.

On nightly a safe implementation can be done by using core::intrinsic::discriminant_value. This is unstable, but it can be avoided to use a feature in the top-level crate by using #[internal_unstable].

This is also how std implements PartialOrd and Ord.

Obviously has to be hidden behind a crate feature.

[ModProg]

Does this need to be hidden behind a feature, or should we check for nightly?

[daxpedda]

Is it possible to check for nightly? The only thing I know of is this RFC: rust-lang/rust#64796.

[ModProg]

you are correct, probably we should use a feature then, because this does not look very ideal:
https://stackoverflow.com/questions/49247618/check-whether-compiler-is-nightly-at-compile-time