Resources can't be put into tags

```
    tag t {
        t0(r);
    }

    let i = @mutable 0;
    {
        let a <- t0(r(i));
    }
    assert *i == 1;
```

This performs a copy of r(i). With other types, e.g. `{a: r(i)}` trans optimizes this to a move, so kind analysis allows it.
