Merge some of the Round methods

[fjarri]

Round has several methods that deal with the round's position in the state transition graph:

• id(&self) -> RoundId,
• fn possible_next_rounds(&self) -> BTreeSet<RoundId>,
• with Determining that a round may produce a result #66 fixed, possibly some fn returns_result(&self) -> bool.

Similarly, there are three methods dealing with how the node interacts with other nodes during the round (intersects with #4):

• fn message_destinations(&self) -> &BTreeSet<Id>,
• fn expecting_messages_from(&self) -> &BTreeSet<Id>,
• to be added in Logic fixes for rounds with partial echo #67, fn echo_round_participation(&self) -> EchoRoundParticipation<Id>.

Does it make sense to merge either of the group into one method returning an enum? Which possibilities do we need to cover?