Move lint rules out of `checkers/ast/mod.rs` (#5957)
## Summary
This PR attempts to draw some basic separation between the `Checker`'s
traversal responsibilities (traversing the AST, building the semantic
model) and its calling-out-to-lint-rule responsibilities. It doesn't try
to introduce any sophisticated API. Instead, it just moves all of the
lint rule calls out of `checkers/ast/mod.rs` and into methods in a new
`analyze` module. (There are four remaining lint rules in `Checker`, but
I'll remove those in future PRs.)
I'm not trying to "solve" our lint rule API here. Instead, I'm trying to
make two improvements:
1. `checkers/ast/mod.rs` has just gotten way too large, and people work
in it all the time. Prior to this PR, it was 5.5k lines, which led to
significant lags in my editor and made it really hard to reason about
the parts that are _actually_ important. (I like big files, but this one
crossed the line for me.) Now, it's < 2,000 lines, and the code is much
more focused.
2. I want to avoid accidentally adding lint rules in the "wrong" parts
of the traversal. By confining lint rule invocations to these "analyze"
calls, we'll avoid (e.g.) putting them in the binding phase.
