Policy of supported platform version for constant values

[gnzlbg]

We have many bugs where the value of a constant does not match the value of the constant in the users system.

I think that, at least for constants, we should support the latest released version of the platform, and that we do not consider upgrading a constant value a breaking change.

If the libc crate ever gets support for multiple versions of the same platform, we should revisit this policy.

[alexcrichton]

I agree! For now upating constants as they're updated on the system hasn't caused problems and seems like we should continue to do so