RFC: Macro reform

[Rendered view](https://github.com/kmcallister/rfcs/blob/macro-reform/text/0000-macro-reform.md)
I don't think we should introduce language syntax for macros - it is clear at the moment that everything macro-like uses its own syntax which is separate from macro-free Rust. I prefer to keep it that way, it makes macro systems more pluggable and I believe will make things easier to understand.