RFC: Macro reform

[Rendered view](https://github.com/kmcallister/rfcs/blob/macro-reform/text/0000-macro-reform.md)
Basically I see it as

- force most people to write `macro_rules!`, for years
- force some people to write `#[feature(improved_macros)]` or `#[version(2)] macro!`, if and when such back-incompat enhancements are developed