Reduce size of Expr from 80 to 64 bytes (#9900)

## Summary

This PR reduces the size of `Expr` from 80 to 64 bytes, by reducing the
sizes of...

- `ExprCall` from 72 to 56 bytes, by using boxed slices for `Arguments`.
- `ExprCompare` from 64 to 48 bytes, by using boxed slices for its
various vectors.

In testing, the parser gets a bit faster, and the linter benchmarks
improve quite a bit.

crates/ruff_python_ast/src/visitor/transformer.rs

@@ -448,10 +448,10 @@ pub fn walk_expr<V: Transformer + ?Sized>(visitor: &V, expr: &mut Expr) {
             range: _,
         }) => {
             visitor.visit_expr(left);
-            for cmp_op in ops {
+            for cmp_op in &mut **ops {
                 visitor.visit_cmp_op(cmp_op);
             }
-            for expr in comparators {
+            for expr in &mut **comparators {
                 visitor.visit_expr(expr);
             }
         }
@@ -580,10 +580,10 @@ pub fn walk_arguments<V: Transformer + ?Sized>(visitor: &V, arguments: &mut Argu
     // Note that the there might be keywords before the last arg, e.g. in
     // f(*args, a=2, *args2, **kwargs)`, but we follow Python in evaluating first `args` and then
     // `keywords`. See also [Arguments::arguments_source_order`].
-    for arg in &mut arguments.args {
+    for arg in arguments.args.iter_mut() {
         visitor.visit_expr(arg);
     }
-    for keyword in &mut arguments.keywords {
+    for keyword in arguments.keywords.iter_mut() {
         visitor.visit_keyword(keyword);
     }
 }