RFC: Macro reform

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As I stated on #440, I don't like taking this nice name for this imperfect macro system. It seems unfortunate to me that one would have to explicitly opt-in to some new improved macro system if/when we create a parallel one that incorporates the backwards-incompatible improvements to `macro_rules!`. (That is, it is extremely unfortunate that the obvious `macro! foo { ... }` invocation would give the *deprecated* form.)