Rewrite Scala codegen to handle all data as traits (#686)

* Rewrite Scala codegen to return data as traits

* Fix possible types check

* Update unit tests

* Properly handle deep fields inside fragments

* Emit Scala types for variables

* Fix default value

* Fix input tracking

* Fix variables input type

* Use library defined optional types

* Update snapshots

* Update snapshots

* Completely drop case classes from codegen

* Don't hardcode None for OptionalInput

* Minimize diff relative to master

Author: shadaj