RFC: Macro reform

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Macro-defining macros are too useful to drop on the floor, and we don't want to get locked into backwards-compatibility with a system that makes them awkward. They're also not as scary as they sound; in particular, the macro resolver already has to deal with a partially-expanded tree, and in the standard way of doing this (expand the tree in left-to-right order, and the resolver only gets to look at definitions that are to the left of the current spot), macro-defining macros Just Work.