Disallow return types of `&'static foo` from being written as `&foo`

As far as I can tell, when `&str` appears as the return type of a function, this silently implies `&'static str`. I think this holds for all other types as well, not just `str`. I propose that we force people to be explicit here, since our general rule is that any borrowed pointer that is returned from a function must have an explicit lifetime.
