Byron CDDLs are missing transaction encodings

At first I thought the `tx` type would be the transaction type in Byron, but it turns out it is not. The `GenTx` for Byron is in fact a:
```haskell
data AMempoolPayload a
= -- | A transaction payload (transaction and witness).
MempoolTx !(ATxAux a)
| -- | A delegation certificate payload.
MempoolDlg !(Delegation.ACertificate a)
| -- | An update proposal payload.
MempoolUpdateProposal !(Update.AProposal a)
| -- | An update vote payload.
MempoolUpdateVote !(Update.AVote a)
```

where each of the options has a different tag and contents. It would be necessary to add these definitions to the cddl file:

```cddl
transaction = [0, [byron.tx, [+ byron.twit]]] ; MempoolTx
/ [1, ...]
/ [2, ...]
/ [3, ...]
```