Renaming inside of clone is very brittle

At the moment `clone` seems to only handle single `ids` and this also seems brittle as witnessed by the failing tests in `example/uniqueness.gr` and for example that the following fails:

```
unitR : forall {a : Type, id : Name} . !(*(Ref id a)) -> exists {id1 : Name} . !(Ref id1 a)
unitR t = clone t as x in share x
```