RFC: Stabilize std::prelude

Stabilize the `std::prelude` module by removing some of the less commonly used
functionality of it.
typo
Add a bit about a prelude::v1 module
Merge branch 'prelude' of https://github.com/alexcrichton/rfcs into alexcrichton-prelude
RFC 503 is prelude stabilization
Merge branch 'alexcrichton-prelude' into HEAD