RFC: numerics reform

This RFC is preparation for API stabilization for the `std::num` module.  The goal is to finish the simplification efforts started in [@bjz's reversal of the numerics hierarcy](https://github.com/rust-lang/rust/issues/10387).

Broadly, the proposal is to collapse the remaining numeric hierarchy in `std::num`, and to provide only limited support for generic programming (roughly, only over primitive numeric types that vary based on size).  Traits giving detailed numeric hierarchy can and should be provided separately through the Cargo ecosystem.

Thus, this RFC proposes to flatten or remove most of the traits currently provided by `std::num`, and generally to simplify the module as much as possible in preparation for API stabilization.

[Rendered](https://github.com/aturon/rfcs/blob/num-reform/active/0000-num-reform.md)