Reduce size of Stmt from 144 to 120 bytes (#11051)

## Summary

I happened to notice that we box `TypeParams` on `StmtClassDef` but not
on `StmtFunctionDef` and wondered why, since `StmtFunctionDef` is bigger
and sets the size of `Stmt`.

@charliermarsh found that at the time we started boxing type params on
classes, classes were the largest statement type (see #6275), but that's
no longer true.

So boxing type-params also on functions reduces the overall size of
`Stmt`.

## Test Plan

The `<=` size tests are a bit irritating (since their failure doesn't
tell you the actual size), but I manually confirmed that the size is
actually 120 now.

crates/ruff_python_formatter/src/statement/clause.rs

@@ -108,7 +108,7 @@ impl<'a> ClauseHeader<'a> {
                 returns,
                 body: _,
             }) => {
-                if let Some(type_params) = type_params.as_ref() {
+                if let Some(type_params) = type_params.as_deref() {
                     visit(type_params, visitor);
                 }
                 visit(parameters.as_ref(), visitor);