Superkinds

For #7083.

The metadata issue with the old version is now fixed. Ready for review.

This is also not the full solution to #7083, because this is not supported yet:
```
trait Foo : Send { }

impl <T: Send> Foo for T { }

fn foo<T: Foo>(val: T, chan: std::comm::Chan<T>) {
    chan.send(val);
}
```

cc @nikomatsakis