RFC: Generic conversion traits

This RFC proposes several new *generic conversion* traits. The
motivation is to remove the need for ad hoc conversion traits (like
`FromStr`, `AsSlice`, `ToSocketAddr`, `FromError`) whose *sole role*
is for generics bounds. Aside from cutting down on trait
proliferation, centralizing these traits also helps the ecosystem
avoid incompatible ad hoc conversion traits defined downstream from
the types they convert to or from. It also future-proofs against
eventual language features for ergonomic conversion-based overloading.

[Rendered](https://github.com/aturon/rfcs/blob/conversion-traits/text/0000-conversion-traits.md)