Derived `Hash` impl is duck-typed

The following compiles successfully with `rustc 1.0.0-nightly (170c4399e 2015-01-14 00:41:55 +0000)`:

``` rust
struct Bar;
impl Bar {
    fn hash<T>(&self, _: T) {}
}
#[derive(Hash)]
struct Foo(Bar);
```

The derived code should probably use UFCS.
