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/cast.rs

@@ -0,0 +1,16 @@
+use rustpython_parser::ast::{Expr, Stmt, StmtKind};
+
+pub fn name(stmt: &Stmt) -> &str {
+    match &stmt.node {
+        StmtKind::FunctionDef { name, .. } | StmtKind::AsyncFunctionDef { name, .. } => name,
+        _ => panic!("Expected StmtKind::FunctionDef | StmtKind::AsyncFunctionDef"),
+    }
+}
+
+pub fn decorator_list(stmt: &Stmt) -> &Vec<Expr> {
+    match &stmt.node {
+        StmtKind::FunctionDef { decorator_list, .. }
+        | StmtKind::AsyncFunctionDef { decorator_list, .. } => decorator_list,
+        _ => panic!("Expected StmtKind::FunctionDef | StmtKind::AsyncFunctionDef"),
+    }
+}