Commit-and-reveal abstraction

In many (most?) cases when private information/result is returned to a the transaction requester, it should be only revealed _after_ the transaction is committed (or the requester might cheat, e.g., pretend to close an auction to learn the winner bid without really committing to the closure). To remind programmers about this important (but subtle) aspect but also to make their life easier, we might want to add a corresponding abstraction to the programming model, e.g., a _committed_return_ in addition to the normal _return_.

Implementation-wise, a potential way would be keep in the cc-(metadata)-state a list of such commited returns as a and then allow _queries_ to retrieve the result (conditioned it is the same requestor).

(Related to issue #83 and #150)