Representation of oneof with tuples

[pallix]

The current representation of oneof field use tuples. This is problematic when using the Poison library to serialize a Protobuf structure to JSON: tuples cannot be converted to JSON (only arrays, maps etc.).

Would it make sense and would it be possible to use an array (possibly optionally if it breaks code) to represent oneof fields? This would allow a great interoperability between exprotobuf and JSON.