Rework rust visitor infrastructure

[Nadrieril]

Right now we're using the derive_visitor crate for our visitors. This has a few limitations I've been running into lately:

• No support for generics: finding all the uses of RegionBinder to track binding levels is error-prone;
• No support for composing visitors: I'd like an easy way to make a visitor wrapper that adds binding level information to another visitor, or span information;
• No control over visits: to avoid traversing all Tys always I use the visit_inside_ty hack today; this is regularly annoying. Moreover in some cases I'd like to stop recursing into a specific variant or similar control.

I'm in the process of thinking through how to solve these problems without having to implement visitors manually for each of our types.