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).
We need to perform full resolution (e.g. method resolution) to determine all APIs used by a library, and this ends up amounting to basically a full compilation. It's a good smoke test regardless to compile before publishing!

Due to needing this analysis I don't think we can ignore `cfg` as if we enabled everything we'd probably get compilation errors due to multiple definitions of the same item in a library.