Can't define types as resources

The following doesn't work:

```
type rtype = resource(int i);
```

Then I would expect to be able to:

```
resource r1(int i) {};
resource r2(int i) {};

let rtype i1 <- r1(0);
let rtype i2 <- r2(0);
```

Currently I don't see any way to make polymorphic resources.
