Flip serialization of `TxIx` in MemPack

[jasagredo]

For the lexicographic order on the serialized form of TxIns to match the Haskell Ord, we need to serialize the TxIx inside in big-endian. We implemented this in consensus in IntersectMBO/ouroboros-consensus#1652, but this change should be upstreamed to the ledger.