~Foo should auto-borrow to &Foo when casting to a trait object

It'd be nice if this would work:

``` rust
trait Foo {}
impl<'self> Foo for &'self str {}
let a = ~"";
let b = &a as &Foo;
```

You currently have to manually borrow

``` rust
trait Foo {}
impl<'self> Foo for &'self str {}
let a = ~"";
let b: &str = a;
let b = &b as &Foo;
```
