Add function `isProperSubsetOf` to module `UTxO`.
This function is analogous to `Map.isProperSubmapOf`. Before this change, we would have to write: ``` (someUTxO `UTxO.isSubSetOf` someOtherUTxO) && (someUTxO /= someOtherUTxO) ``` After this change, we can write: ``` someUTxO `UTxO.isProperSubsetOf` someOtherUTxO ```