std::fmt::Pointer should be implemented for pointers to T: ?Sized

Example:

``` rust
fn main() {
    let x: Box<std::fmt::Debug> = Box::new("hello");
    println!("{:p}", x);
    // error: the trait `core::marker::Sized` is not implemented for the type `core::fmt::Debug`
}
```

I think Pointer should be impled as follows:

```
impl<T: ?Sized> Pointer for Box<T>
```

and likewise for borrowed refs, raw pointers, Arc etc.
