Box type params and arguments fields on the class definition node (#6275)

## Summary

This PR boxes the `TypeParams` and `Arguments` fields on the class
definition node. These fields are optional and often emitted, and given
that class definition is our largest enum variant, we pay the cost of
including them for every statement in the AST. Boxing these types
reduces the statement size by 40 bytes, which seems like a good tradeoff
given how infrequently these are accessed.

## Test Plan

Need to benchmark, but no behavior changes.

crates/ruff_python_parser/src/python.rs

@@ -1,5 +1,5 @@
 // auto-generated: "lalrpop 0.20.0"
-// sha3: aadf067e37a9f39d450f1403b759a9659c60e697758ddd2b8c2b5fa2d0d73672
+// sha3: f99d8cb29227bfbe1fa07719f655304a9a93fd4715726687ef40c091adbdbad5
 use num_bigint::BigInt;
 use ruff_text_size::TextSize;
 use ruff_python_ast::{self as ast, Ranged, MagicKind};
@@ -33583,10 +33583,10 @@ fn __action170<
         ast::Stmt::ClassDef(
             ast::StmtClassDef {
                 name,
-                arguments,
+                arguments: arguments.map(Box::new),
                 body,
                 decorator_list,
-                type_params,
+                type_params: type_params.map(Box::new),
                 range: (location..end_location).into()
             },
         )