type variables in method names are resolved in wrong scope

This does not work:

```
iface clam<A> { }


impl foo<A> for [A] {
   fn bar<B,C:clam<B>>(c: C) -> B { fail; }
}

fn main() { }
```

It yields:

```
/Users/nmatsakis/tmp/iface-res-bug.rs:5:19: 5:20 error: unresolved typename: B
/Users/nmatsakis/tmp/iface-res-bug.rs:5    fn bar<B,C:clam<B>>(c: C) -> B { fail; }
```
