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).
Will the "cycles" mentioned in this be six weeks long?

During the 1.0.0-beta period, will there exist separate beta and nightly branches, or will all development be on 1.0.0-beta, with changes reduced to those that are non-catastrophic?