## Summary
Type visitors are conceptually immutable, they just internally track the
types they've seen (and some maintain a cache of results.) Passing
around mutable visitors everywhere can get us into borrow-checker
trouble in some cases, where we need to recursively pass along the
visitor inside more than one closure with non-disjoint lifetime.
Use interior mutability (via `RefCell` and `Cell`) inside the visitors
instead, to allow us to pass around shared references.
## Test Plan
Existing tests.
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