Enums cannot have default values (initializer)

[pataei]

In enum well-formedness rule: Petr4 doesn't allow for an enum to have default values (initializer) for constants, however, P4 spec does. Here's an example.

[pataei]

Wrong! Here's the code:

    | SerializableEnum of
        { tags: 'a;
          annotations: Annotation.t list;
          typ: Type.t [@key "type"];
          name: P4String.t;
          members: (P4String.t * Expression.t) list }