Add instance Uninhabited _|_

I'm playing around but seems like I should be able to write this:

```
false : _|_ -> a
false = absurd
```

But we don't have an `Uninhabited` instance for `_|_` - the easiest one:

```
instance Uninhabited _|_ where
  uninhabited a = a
```

The implementation is just `id`!