Compiler message need improve for OrdSliceExt::sort()

Code:

```
// #[deriving(Eq, Ord, PartialEq, PartialOrd)]
struct X { x: int }

pub fn main() {
    let mut b: Vec<X> = Vec::new();
    b.sort();
}
```

Error:

```
tmp.rs:6:7: 6:13 error: type `collections::vec::Vec<X>` does not implement any method in scope named `sort`
tmp.rs:6     b.sort();
```

The compiler should tell us, struct `X` doesn't implements `Ord`, at least.
