Unboxed trait object can be stored in struct without lifetime

``` rust
trait Foo { fn method(&self); }

struct Wrapper(Foo);

fn main() {}
```

It seems to me that this should be requiring a lifetime, e.g. `struct Wrapper<'a>(Foo + 'a);` or `struct Wrapper(Foo + 'static);`. However, it may be the intended behaviour too, nominating for clarification.
