Release channels and feature staging

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.

[Rendered](https://github.com/brson/rfcs/blob/release-channels/text/0000-release-channels.md).