RFC: Macro reform

[Rendered view](https://github.com/kmcallister/rfcs/blob/macro-reform/text/0000-macro-reform.md)
Is writing `macro_rules!` really that horrible? For me, the details of the "keyword" just fades into the background.

Long term, `improved_macros` are what people are going to use, so it seems highly unfortunate to hamstring that for the sake of saving some typing in the short-term. If it turns out that we have no interest/no backwards-compatible changes, we can promote it to `macro!` then (with the appropriate aliasings for backwards compatibility).

I would be very happy to make the other improvements listed here and then reevaluate renaming to `macro!` after using it for a while & getting feedback.