Add type for returning arbitrary size tuples

The `postgresql-simple` library has a type `:.` that facilitates returning query results of arbitrary size.

https://hackage.haskell.org/package/postgresql-simple-0.6.2/docs/Database-PostgreSQL-Simple.html#t:Only

(I didn't link to `:.` because GitHub doesn't handle it properly)

Could something similar be added to beam? I'm not stuck on `:.` as the operator. Servant has a `:.` operator that conflicts, so I'm open to anything. But the convenience would be really nice.