PrimUnlifted class

I'd like to move this from `primitive-unlifted` into this `primitive`:

```
class PrimUnlifted a where
type Unlifted a :: UnliftedType
toUnlifted# :: a -> Unlifted a
fromUnlifted# :: Unlifted a -> a

instance PrimUnlifted (Array a) where
type Unlifted (Array a) = Array# a
toUnlifted# (Array a) = a
fromUnlifted# x = Array x

instance PrimUnlifted (MutableArray s a) where
type Unlifted (MutableArray s a) = MutableArray# s a
toUnlifted# (MutableArray a) = a
fromUnlifted# x = MutableArray x

instance PrimUnlifted (SmallArray a) where
type Unlifted (SmallArray a) = SmallArray# a
toUnlifted# (SmallArray a) = a
fromUnlifted# x = SmallArray x

instance PrimUnlifted (SmallMutableArray s a) where
type Unlifted (SmallMutableArray s a) = SmallMutableArray# s a
toUnlifted# (SmallMutableArray a) = a
fromUnlifted# x = SmallMutableArray x

instance PrimUnlifted (PrimArray a) where
type Unlifted (PrimArray a) = ByteArray#
toUnlifted# (PrimArray x) = x
fromUnlifted# x = PrimArray x

instance PrimUnlifted ByteArray where
type Unlifted ByteArray = ByteArray#
toUnlifted# (ByteArray x) = x
fromUnlifted# x = ByteArray x
```

I'd have to give up on the `ShortText` and `ShortByteString` instances since `primitive` is not going to pick up `bytestring` or `text-short` as a dependency. Having this typeclass would let us have a much more useful implementation of `touch` and `keepAlive`. Any implementation of these targeting a lifted wrapper is dangerous and misleading to the user. Unfortunately, that's basically the *only* thing we would get from doing this. All the other stuff using this class needs to stay in `primitive-unlifted` for at least a while longer since the backwards-compatibility story for it is rather bad. I do think that `PrimUnlifted` is stable at this point though.

Any thoughts on this?