CDDL:Conway: Registration-delegation certificates (indexed 11, 12 and 13) should be combined into a single definition

We need something like
```haskell
account_registration_delegation_cert :: Named Group
account_registration_delegation_cert = "account_registration_delegation_cert" =:~
  grp [11, a stake_credential, a pool_keyhash, a coin] /
  grp [12, a stake_credential, a drep, a coin] /
  grp [13, a stake_credential, a pool_keyhash, a drep, a coin]
```