Create `To/FromPersistValues` classes

In #1321 I discussed a new pair of type classes. THe first one is easy:

class ToPersistValues a where
    toPersistValues :: a -> [PersistValue]

But the second is a bit more tricky.

class FromPersistValues a where
    fromPersistValues :: RowParser a

newtype RowParser a 
    = RowParser 
    { unRowParser :: ExceptT Text (State [PersistValue]) a
    }

I think that's a good way to approach it - we'd ideally want to be able to have instances like:

instance (FromPersistValues a, FromPersistValues b) => FromPersistValues (a, b) where
     fromPersistValues = (,) <$> fromPersistValues <*> fromPersistValues 

Start by reading issue #1321 for the discussion behind the proposed ToPersistValues and FromPersistValues classes, then inspect the existing persistence and RowParser APIs. Done means the two classes and the proposed tuple-style composition approach are defined consistently with the project’s existing types.