Handle cases where a node sent or not sent a part of the message unexpectedly

[fjarri]

Fixes #23

The gist of the changes is that now even if a node does not send, say, a direct message, it still signs a None value with the corresponding metadata and sends it off in a bundle with the rest of the parts. This way the receiver can assert that the direct part or the echo part should be none, and register a provable error if they aren't.

Of course the node could just not send the corresponding part at all (equivalent to sending any other malformed or mal-signed message), but as #39 suggests, we're mostly protecting against nodes with obsolete software, not actively malicious ones.

Changes:

• Changed the signatures of Round::make_echo_broadcast(), Round::make_direct_message(), Round::receive_message() removing Options there.
• Added Round::make_direct_message_with_artifact(); Round::make_direct_message() would be the one most used because most rounds in Synedrion don't actually create an artifact.
• Artifact::empty() removed, since now we can just return None.

Outstanding questions:

• We do not need to keep signed empty messages in the transcript, but there is currently no type system assertion for that. Perhaps we don't need one, since the payload being None leads to the same outcome as a deserialization error on a Some payload, so DirectMessage::deserialize() just returns a DirectMessageError in this case as well, same for the echo broadcast.
• The fallback from make_direct_message_with_artifact() to make_direct_message() is a little tricky in RoundOverride; if the round defines the former, but we override the latter, the override won't be effective. Not sure how to handle that; I really want to keep the artifact-creating method separate since it's not used all that much.
• Should we also allow returning None for an empty Payload?

[coveralls]

Pull Request Test Coverage Report for Build 11524276230

Details

• 118 of 179 (65.92%) changed or added relevant lines in 9 files are covered.
• 14 unchanged lines in 4 files lost coverage.
• Overall coverage decreased (-0.8%) to 70.528%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
manul/src/testing/macros.rs	15	17	88.24%
manul/src/session/session.rs	19	22	86.36%
manul/src/protocol/errors.rs	1	8	12.5%
manul/src/protocol/round.rs	6	13	46.15%
manul/src/session/evidence.rs	6	14	42.86%
manul/src/protocol/message.rs	40	74	54.05%

Files with Coverage Reduction	New Missed Lines	%
manul/src/session/message.rs	1	98.68%
manul/src/protocol/round.rs	3	60.34%
manul/src/session/session.rs	4	72.59%
manul/src/testing/macros.rs	6	87.18%

Totals
Change from base Build 11514244461:	-0.8%
Covered Lines:	1242
Relevant Lines:	1761

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💛 - Coveralls