pgtype.Row() was optimized for a single line use
without much ceremony at a cost of OID registration,
which is cumbersome. In practice it so much incovnenience
to create new Composite just before making a query.
So now there is just a Composite type and 2 helper methods:
- SetFields sets composite fields to values passed. This assignment
fails if types passed are not assignable to Values pgtype is
made of.
- Scan acts exactly like query.Scan, but for a composite value. Passed
values are set to values from SQL composite.