Adding clone functions for all operation_t types

[MarcusJGStreets]

For #285

[athoelke]

Although it is possible to hypothesize use cases for clone functionality on all these operation types, I'd prefer to only add such functionality to the operation types where there is a demonstrable use case and value.

In this case, it would be to just add this functionality to MAC - where there is no trade-off with security (no random IV or unique nonce, which should not be cloned).