RFC: Macro reform

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> Is writing `macro_rules!` really that horrible? For me, the details of the "keyword" just fades into the background.

Well, I think it is, but that's more a matter of taste than anything else.

It certainly makes one wonder what other `macro_*!` forms there are, and the answer right now is "other things that don't exist yet and will be better in unspecified ways", which is not really that satisfying for a "stable" feature.

I just don't think we should stabilize a feature but discourage its use with ugly names.  It's a mixed message for sure.

> 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.

I'm okay with this but there's just not much time left before 1.0.  Fortunately `macro_rules!` → `macro!` would be a simple sed, so it's fairly reasonable to do it as a last minute pre-1.0 change.