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_parser/src/parser/statement.rs

@@ -1738,7 +1738,7 @@ impl<'src> Parser<'src> {
 
         ast::StmtFunctionDef {
             name,
-            type_params,
+            type_params: type_params.map(Box::new),
             parameters: Box::new(parameters),
             body,
             decorator_list,