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/hashable.rs

@@ -0,0 +1,40 @@
+use std::hash::Hash;
+
+use rustpython_parser::ast::Expr;
+
+use crate::comparable::ComparableExpr;
+
+/// Wrapper around `Expr` that implements `Hash` and `PartialEq`.
+pub struct HashableExpr<'a>(&'a Expr);
+
+impl Hash for HashableExpr<'_> {
+    fn hash<H: std::hash::Hasher>(&self, state: &mut H) {
+        let comparable = ComparableExpr::from(self.0);
+        comparable.hash(state);
+    }
+}
+
+impl PartialEq<Self> for HashableExpr<'_> {
+    fn eq(&self, other: &Self) -> bool {
+        let comparable = ComparableExpr::from(self.0);
+        comparable == ComparableExpr::from(other.0)
+    }
+}
+
+impl Eq for HashableExpr<'_> {}
+
+impl<'a> From<&'a Expr> for HashableExpr<'a> {
+    fn from(expr: &'a Expr) -> Self {
+        Self(expr)
+    }
+}
+
+impl<'a> HashableExpr<'a> {
+    pub const fn from_expr(expr: &'a Expr) -> Self {
+        Self(expr)
+    }
+
+    pub const fn as_expr(&self) -> &'a Expr {
+        self.0
+    }
+}