Add lint for unused lifetimes

A step towards  #12805.

 This could be extended to warn for things like this as well (either in this or in another PR):
```
struct Foo<'a> {
    a: int
}
```
As discussed on IRC, this is needed since there is no proper Self in Arg (related to [this](https://github.com/rust-lang/rust/blob/master/src/libsyntax/ast.rs#L825) comment).