Create a rust_python_ast crate (#3370)

This PR productionizes @MichaReiser's suggestion in https://github.com/charliermarsh/ruff/issues/1820#issuecomment-1440204423, by creating a separate crate for the `ast` module (`rust_python_ast`). This will enable us to further split up the `ruff` crate, as we'll be able to create (e.g.) separate sub-linter crates that have access to these common AST utilities.

This was mostly a straightforward copy (with adjustments to module imports), as the few dependencies that _did_ require modifications were handled in #3366, #3367, and #3368.

crates/ruff_python_ast/src/lib.rs

@@ -0,0 +1,17 @@
+pub mod branch_detection;
+pub mod cast;
+pub mod comparable;
+pub mod context;
+pub mod function_type;
+pub mod hashable;
+pub mod helpers;
+pub mod logging;
+pub mod operations;
+pub mod relocate;
+pub mod source_code;
+pub mod strings;
+pub mod types;
+pub mod typing;
+pub mod visibility;
+pub mod visitor;
+pub mod whitespace;