Simplify public-facing message API

[fjarri]

• re-export digest module from session (should have been done when Digest was added to SessionParameters)
• rename *MessageBundle to *Message, and Signed/VerifiedMessage to Signed/VerifiedMessagePart. Fixes Simplify public-facing type names #42
• make VerifiedMessage generic over Verifier and not the whole SessionParameters
• add Message::destination() to simplify the user's life a bit.

[coveralls]

Pull Request Test Coverage Report for Build 11600047451

Details

• 54 of 61 (88.52%) changed or added relevant lines in 6 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.03%) to 71.184%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
manul/src/session/transcript.rs	6	7	85.71%
manul/src/session/evidence.rs	4	7	57.14%
manul/src/session/message.rs	18	21	85.71%

Files with Coverage Reduction	New Missed Lines	%
manul/src/session/evidence.rs	1	41.03%

Totals
Change from base Build 11599314557:	0.03%
Covered Lines:	1497
Relevant Lines:	2103

---

💛 - Coveralls