Add support for narrowing a doobie row based on a discriminator

[milessabin]

GraphQL supports subtype polymorphism via interface extension and safe downcasting using fragments. Currently for a doobie backed model this has to be implemented via subqueries, ie. we first have to read a discriminator value and then use that to create a subtype specific continuation query.

We should be able to support inlining small sets of subtypes in a single row with a discriminator to avoid having to make an additional query.

[milessabin]

Fixed in #152 and #188.