feat(map): Codegen for GraphQL Target

[jonathan-casey]

Sample

type Map1 {
   key: String
   value: String
}
type Map2 {
   key: String
   value: DateTime
}
type Map3 {
   key: String
   value: SSN
}
type Map4 {
   key: String
   value: Concept
}
type Map5 {
   key: SSN
   value: String
}
type Map6 {
   key: SSN
   value: Employee
}

[coveralls]

coverage: 98.897% (+0.004%) from 98.893% when pulling d826e9a on jonathan/codegen_map_graphql into 64de665 on main.