RFC: Stabilize std::prelude

Stabilize the `std::prelude` module by removing some of the less commonly used
functionality of it.

[Rendered](https://github.com/alexcrichton/rfcs/blob/prelude/text/0000-prelude-stabilization.md)
Are we insisting that this will never change by promising the existence of these traits forever? I was under the impression that we wanted to fix this at some point.