New `TxOut` definition can be refactored

[Jimbo4350]

data TxOut ctx era where
  TxOut :: L.EraTxOut era => L.TxOut era -> Maybe (Datum ctx era) -> TxOut ctx era

The datum (or lack thereof) can be added to the L.TxOut era value. I.e we should not need a separate Maybe (Datum ctx era) field. Explore this.