Omitting an lifetime name in the return value of an function should be always forbidden.

Right now any function signature of the form

```
fn foo(...) -> &Bar
```

is valid, _but only if the thing returned has the static lifetime_.

I think it would be less confusing if we would just always forbid that, requiring to write out the `'static` explicitly in those cases:

```
fn foo(...) -> &'static Bar
```
