Move binding and scope tracking into a separate ast::Context struct

[charliermarsh]

This PR is an attempt to split up Checker in such a way so as to allow us to break the Ruff linter itself into multiple crates.

The first step (seen here) is to remove all AST-visitation state into a new Context struct, which tracks the current scope stack, available bindings, etc. By moving these fields and methods off of Checker, rules can instead take Context as an argument, rather than Checker, which means they can be decoupled from the actual "driver" / "visitor" and thereby the rest of the lint rules.

[charliermarsh]

Diagnostic is another tough cycle to break. Depends on DiagnosticKind which depends on all rule violations. Need to play around with this and reconsider the struct hierarchy.

[charliermarsh]

Settings is another. Perhaps we can change rules to rely on the settings they care about, rather than the Settings struct, and rely on Checker to inject those settings at the call-site.

[charliermarsh]

Okay, this can be merged as-is (in that, it doesn't depend on any additional refactors to be useful).

[JonathanPlasse]

How much did the compilation time improved?
Or is it preparation for future improvements?

[MichaReiser]

LGTM: Something that's unclear to me but isn't something that we need to solve now is how do I decide whether to add some logic to Checker or Context? It is my impression that there are still many methods on Checker that only use state stored on the Context. Should these be moved to Context at some point?

[MichaReiser]

Nit: Can you add some documentation explaining what a Deferred is? I'm unable to guess its usage from just its name.

[charliermarsh]

It is my impression that there are still many methods on Checker that only use state stored on the Context. Should these be moved to Context at some point?

The intent is that these should all be moved over to Context. Lemme see if I missed any...

[charliermarsh]

How much did the compilation time improved? Or is it preparation for future improvements?

Yeah just in preparation for future improvements. I don't expect this alone to do much, it's more that it sets us up to be able to split the crate later.