Release channels take 2

This RFC describes changes to the Rust release process, primarily the division of Rust's time-based releases into 'release channels', following the 'release train' model used by e.g. Firefox and Chrome; as well as 'feature staging', which enables the continued development of experimental language features and libraries APIs while providing strong stability guarantees in stable releases.

This revision includes a significant simplification by merging stability attributes with feature gates, which further enables some additional useful features within Cargo related to version detection. One big implication of this change is that stability attributes are no longer suitable for use outside of std (if such a feature is desirable it is intended to be solved by Cargo).
I would expect us to create a blanket "1.0" feature which represents the initial release and we'd just tag all stability attributes as being part of that category.

I also think that `#[unstable]` for "library design is hard" is fine because it legitimately is! We'd probably phrase it along the lines of "unsure whether this will be the final design" or something like that.