Adds a test (and necessary infrastructure) for serialisation and
deserialisation of Alonzo-era TxBody.

- Introduce the transaction type for Alonzo
- Introduce serialisation code for transactions. Note that currently,
  the transaction serialisation does _not_ allow Shelley/Allegra/Mary
  era transactions to decode directly as Alonzo transactions. If we
  decide that we should do this, it should not be particularly difficult
  to allow this, but since we do not sign transactions directly, we may
  simply convert old to new transactions and know that the TxBody will
  still have the correct hash.
- Add a round-trip test for Tx serialisation.