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 suspect that having compiler assistance in doing so will be a great boon to authors as I highly doubt that we'll take great care to make sure all our libraries mention a version they're compatible with. Not only that but this helps automate the entire process and smooth out the publishing story rather than getting an error saying "you must specify a rustc version" when publishing.