Elision lint should not do anything if trait refs are involved

This is not always elidable, since it's possible to be `&'a Trait + 'a` too. Neither cases allow elision consistently, AFAICT.

``` rust
fn foo<'a>(&'a self) -> &'a Trait {

}
```

`walk_ty`ing and searching for anything wrapping a trait _except_ for a `Box` (`TyUniq`) should work. 
