The derive-visitor crate has a few shortcomings:

• Doesn't support generics, which made it hard to properly account for all the cases where we may encounter Binder<SomeType>.
• Doesn't give control over recursion, which I worked around with the Ty::visit_inside hack (a regular source of confusion).
• Doesn't support exfiltrating visited references out of the visitor, which I have wanted for reconstructing comments.

The last two points could be remedied by adding features to the upstream crate, but the first one requires a completely different approach. Over the last few weeks I came up with a new design for automatically-derived visitors, which I made into a crate over the winter break.

That crate centered around the main boilerplate required for building visitors, namely the code that calls v.visit(self.field) for each field of a type. A derive macro provides that boilerplate, and with that boilerplate out of the way it becomes easy to define flexible visitors. The crate provides more macros for that.

This PR replaces all uses of derive-visitor with a custom visitor infrastructure based on my new derive-generic-visitor crate. This adds a AstVisitable trait implemented for all the types of the AST, as well as a pair of VisitAst[Mut] traits used to traverse such types. They provide a convenient overrideable interface to define visitors, of which you can see examples in this PR. Moreover we still support the extremely practical dynamic-dispatched-based API that we had with derive-visitors.

The second-to-last commit fixes #505.