Add a package containing common GraphQL to Dart serializers

[shyndman]

Postgres types (potentially) requiring serialization:

Core

bit bit varying bytea interval [ fields ] [ (p) ] money time [ (p) ] [ without time zone ] time [ (p) ] with time zone timestamp [ (p) ] [ without time zone ] timestamp [ (p) ] with time zone uuid

Networking

cidr inet macaddr macaddr8

JSON

json jsonb

Geometry

box circle line lseg geometry

Geography

geography

Fixes #195