## Summary
This PR does some non-behavior-changing refactoring of the AST checker.
Specifically, it breaks the `Stmt`, `Expr`, and `ExceptHandler` visitors
into four distinct, consistent phases:
1. **Phase 1: Analysis**: Run any lint rules on the node.
2. **Phase 2: Binding**: Bind any symbols declared by the node.
3. **Phase 3: Recursion**: Visit all child nodes.
4. **Phase 4: Clean-up**: Pop scopes, etc.
There are some fuzzy boundaries in the last three phases, but the most
important divide is between the Phase 1 and all the others -- the goal
here is (as much as possible) to disentangle all of the vanilla
lint-rule calls from any other semantic analysis or model building.
Part of the motivation here is that I'm considering re-ordering some of
these phases, and it was just impossible to reason about that change as
long as we had miscellaneous binding-creation and scope-modification
code intermingled with lint rules. However, this could also enable us to
(e.g.) move the entire analysis phase elsewhere, and even with a more
limited API that has read-only access to `Checker` (but can push to a
diagnostics vector).
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