Uninitialized fields in splitted constructor

The is_obj and is_tup fields are always set to false in Splitted (it seems).

Related discussion: tezos/tezos!5444 (comment 974058685).

The smart constructor raw_splitted could do the following for instance:

let raw_splitted ~json ~binary =
  make
  @@ Splitted                
  {
     encoding = binary;
     json_encoding = json;
     is_obj = is_obj json && is_obj binary;
     is_tup = is_tup json && is_tup binary;
  }