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20aa182235
Implement AST visitors using a derive macro. AST visitors are large and error-prone beasts. This PR attempts to write them using a derive macro. The design uses three traits: `Visitor`, `Visitable`, `Walkable`. - `Visitor` is the trait implemented by downstream crates, it lists `visit_stuff` methods, which call `Walkable::walk_ref` by default; - `Walkable` is derived using the macro, the generated `walk_ref` method calls `Visitable::visit` on each component; - `Visitable` is implemented by `common_visitor_and_walkers` macro, to call the proper `Visitor::visit_stuff` method if it exists, to call `Walkable::walk_ref` if there is none. I agree this is quite a lot of spaghetti macros. I'm open to suggestions on how to reduce the amount of boilerplate code. If this PR is accepted, I believe the same design can be used for the HIR visitor.