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)
If we are able to get inherent methods on primitive types we can revisit this decision, but for now we can't so this is stabilizing on one extension trait per primitive type if necessary.