Box the biggest ast::ItemKind variants

compiler/rustc_parse/src/lib.rs

@@ -4,6 +4,8 @@
 #![feature(bindings_after_at)]
 #![feature(iter_order_by)]
 #![feature(or_patterns)]
+#![feature(box_syntax)]
+#![feature(box_patterns)]
 
 use rustc_ast as ast;
 use rustc_ast::attr::HasAttrs;