Can't implement methods with the same name on non-overlapping generic specializations

``` rust
struct Foo<T>;
impl Foo<int> {
  fn foo() {}
}
impl Foo<f64> {
  fn foo() {}
}
```

```
t.rs:6:2: 6:13 error: duplicate definition of value `foo`
t.rs:6   fn foo() {}
         ^~~~~~~~~~~
t.rs:3:2: 3:13 note: first definition of value `foo` here
t.rs:3   fn foo() {}
         ^~~~~~~~~~~
```

This is frustrating.
