Add combinator for ordering by primary keys

It took us a good bit to figure out how to `orderBy_` the `primaryKey` of some arbitrary table. It would be much better if this were wrapped up in a combinator and documented:

```haskell
...
orderBy_ (allBeamValues descColumnar . primaryKey) $ all_ tbl
where
-- The last two type parameters of QOrd are phantoms
descColumnar :: Columnar' (QExpr syntax (QNested (QNested QBaseScope))) a -> QOrd syntax () ()
descColumnar (Columnar' col) = coerce $ desc_ col
```