message: fix implicit_data_type
diff: @@ -434,7 +434,7 @@ data_type:
  data_type_or_implicit:
  data_type
  | implicit_data_type;
  -implicit_data_type: signing ( packed_dimension )* | ( packed_dimension )+;
  +implicit_data_type: ( signing )? ( packed_dimension )*;
  enum_base_type:
  integer_atom_type ( signing )?
  | integer_vector_type ( signing )? ( packed_dimension )?
  