Add support for simple broadcasts

[fjarri]

Fixes #24

• EchoBroadcast and DirectMessage now use ProtocolMessagePart trait for their methods.
• Added normal broadcasts support in addition to echo ones; signatures of Round methods changed accordingly.
• Added Round::make_normal_broadcast().
• Added processing of a mismatched signature in the echoed broadcast. It is a provable error now.
• Simplified creating an evidence for an echo round error (the TODO about SignedMessage being generic over signatures does not apply anymore)

[coveralls]

Pull Request Test Coverage Report for Build 11563351210

Details

• 150 of 271 (55.35%) changed or added relevant lines in 10 files are covered.
• 8 unchanged lines in 5 files lost coverage.
• Overall coverage increased (+0.3%) to 70.794%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
manul/src/session/message.rs	20	21	95.24%
manul/src/session/transcript.rs	18	21	85.71%
manul/src/protocol/errors.rs	2	10	20.0%
manul/src/protocol/message.rs	28	36	77.78%
manul/src/session/session.rs	22	31	70.97%
manul/src/protocol/round.rs	9	22	40.91%
manul/src/session/echo.rs	7	26	26.92%
manul/src/session/evidence.rs	27	87	31.03%

Files with Coverage Reduction	New Missed Lines	%
manul/src/session/echo.rs	1	65.97%
manul/src/session/session.rs	1	72.71%
manul/src/protocol/errors.rs	2	19.7%
manul/src/session/evidence.rs	2	42.31%
manul/src/protocol/message.rs	2	75.0%

Totals
Change from base Build 11562272589:	0.3%
Covered Lines:	1355
Relevant Lines:	1914

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💛 - Coveralls