I see PostgresKit being the right place to put the Codable stuff. I do think NIOPostgres should still take care of converting PostgresData to Swift types like String and Int, but that can we done with a simple protocol like PostgresDataConvertible. Then, in PostgresKit, Coders can be built on top on top of the convertible protocol.
I will take a stab at this soon (hopefully this week), but I am always happy for PRs :)