Change ItemModifier and ItemDecorator to traits

For convenience, the traits are implemented for the respective bare
functions. Change code from this:

```rust
ItemDecorator(some_function)
// or
ItemModifier(some_other_function)
```
to
```rust
ItemDecorator(box some_function)
// or
ItemModifier(box some_other_function)
```

[breaking-change]

src/test/auxiliary/macro_crate_test.rs

@@ -36,7 +36,7 @@ pub fn plugin_registrar(reg: &mut Registry) {
     reg.register_macro("identity", expand_identity);
     reg.register_syntax_extension(
         token::intern("into_foo"),
-        ItemModifier(expand_into_foo));
+        ItemModifier(box expand_into_foo));
 }
 
 fn expand_make_a_1(cx: &mut ExtCtxt, sp: Span, tts: &[TokenTree])