Can't `use` names to impl them

``` .rs
struct Foo;

mod bar {
    use super::Foo;
    impl Foo {
        fn f() { }
    }
}
```

gives

```
baz.rs:5:10: 5:13 error: found module name used as a type: impl bar::Foo::Foo (id=8)
baz.rs:5     impl Foo {
                  ^~~
```

when `impl super::Foo` works fine.

rustc 0.11.0 (49bc17bfdd7143909aede81652d7d624cecd8a70 2014-07-07 17:16:34 +0000)
