`Hash::hash`'s type parameter should be `?Sized`

The `Hasher` trait is not `Sized`, but `Hash::hash` is defined to take a sized type parameter. This prevents code like the following from working:

``` rust
use std::hash::{Hash, Hasher};

fn bar(hasher: &mut Hasher) {
    1.hash(hasher);
}

fn main() {}
```
