Combining post-interaction updates

[jricher]

§5.1 Continuing After a Completed Interaction: Editor's note:

Other interaction methods, such as a challenge-response cryptographic protocol, would use a similar construct as here, but have different rules. Would it be reasonable to allow them to be combined? Could this be combined further with the "update" method in Section 5.3?

[jricher]

Additional continuation elements can be defined by #436 and the update status is better defined by the state diagram in #427