RFC: Macro reform

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Okay, maybe that's an unfair comparison because HKT are supposed to be back-compat.  But every macro system we ever have in Rust is going to be "imperfect" in some way.  At some point we have to live with the imperfections and improve them in non-breaking ways, which is what 1.0 is all about, but why back away from that idea just in the case of macros?

The practical outcome of your alternative is that Rust programmers are stuck with the ugly name `macro_rules!` for perhaps years and the unstable alternative will be something not descriptively different, like `macro_case!`.