Representation of oneof with tuples

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.

Start by locating how exprotobuf represents oneof fields and how its generated structures interact with Poison serialization. Compare tuple and array representations, check for compatibility concerns raised by existing behavior, and define done as a representation that can be serialized to JSON without breaking supported Protobuf usage.

Written by the indexing model from the issue text.