Compare and unify JSON serialization types

[joswarmer]

Both in the validation package and in the core package, there are type definitions for the JSON serialisation format. They should be reconciled.
We might want to define a separate package for these types, as they are used in validation and in core. And if we move the LionWeb diff from validation to a separate package, it will be used by this package as well.

[dslmeinte]

I think that we need a separate package just for types and (specific, low-level) functions around the serialization format as well.