RFC: Macro reform

[Rendered view](https://github.com/kmcallister/rfcs/blob/macro-reform/text/0000-macro-reform.md)
It seems to me that since _everything_ to do with procedural macros is unspecified for 1.0, unspecifying the order of evaluation is a given. It seems reasonable to then check that this would not cause problems with built-in macros (as kmc points out below) since they are the only procedural macros which will ship with 1.0.

Long term, I agree we must nail this down properly.