Merge Signer/Verifier/Signature/Digest into SessionParameters

[fjarri]

This significantly reduces the generics noise in the session layer, and also seems logical from the API perspective: signing types are basically a parameter supplied by the user to the session level. So Digest should be a part of them too, and not an associated type of Protocol.

The second commit fixes #6. There is a very slight performance regression, about 2%, but I think it is worth it given the simplified API.

Also makes it much easier to implement #33 - so perhaps it should be redone after this goes in.

[coveralls]

Pull Request Test Coverage Report for Build 11431426942

Details

• 152 of 178 (85.39%) changed or added relevant lines in 8 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.1%) to 77.021%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
manul/src/session/echo.rs	16	17	94.12%
manul/src/session/session.rs	46	47	97.87%
manul/src/session/transcript.rs	10	12	83.33%
manul/src/session/evidence.rs	20	42	47.62%

Files with Coverage Reduction	New Missed Lines	%
manul/src/session/session.rs	1	74.05%

Totals
Change from base Build 11420967409:	0.1%
Covered Lines:	1639
Relevant Lines:	2128

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💛 - Coveralls