This way, the delegation includes all the data necessary to verify it.
The proof master key is added as a separate member rather than as the first level, because it is a special level that does not need to be signed by anyone else.
Add a static `Proof::computeIdFromLimitedId` method to avoid duplicating the proof id computation code. Recomputing the proofid from limitedid and master key is now needed in `Delegation::verify`.
Rename a couple of parameters from `newMaster` to `delegatedKey` in `Delegation:addLevel` to avoid confusion, now that the delegation has a `proofMaster` member.
Depends on D9544