Rethink `Unique`

The issue in #1236 illustrates a need to rethink how we handle Uniques.

Ultimately, the problem comes down to conflating two uses of the type:

1. Specifying a unique key for retrieval (eg getBy :: Unique rec -> m (Maybe (Entity rec)))
2. Specifying a uniqueness constraint for conflict (eg upsertBy).

The EntityField rec typ type works great for talking about the fields on an entity, so it seems like we might copy that design.

-- let's factor this out of PersistEntity while we're at it
class PersistEntity rec => PersistEntityUnique rec where
    data UniqueConstraint rec :: Type -> Type

data Unique rec where
    Unique :: UniqueConstraint rec typ -> typ -> Unique rec

it would be nice if we could somehow capture the actual EntityFields in the UniqueConstraint type, too. Then we could reuse things like persistFieldDef :: EntityField rec typ -> FieldDef. But, we can't promote data family constructors, as that "requires dependent types" for some reason.

Start by reading issue #1236 and the current Unique, EntityField, and PersistEntity definitions. The issue names no files or tests; done would require an agreed redesign that separates retrieval keys from conflict constraints and addresses reuse of EntityFields.