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)
Adding non-default methods is backwards-incompatible, but I believe we're going to consider adding a default method a backwards-compatible change to these extension traits.