Fixes for `Session::preprocess_message()`

[fjarri]

• Use a private enum to get rid of unreachable!() in preprocess_message(). Fixes #20
• Return a PreprocessEnum from Session::preprocess_message() instead of an Option. Fixes #43

[coveralls]

Pull Request Test Coverage Report for Build 11600645963

Details

• 17 of 28 (60.71%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.2%) to 71.354%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
manul/src/session/session.rs	16	27	59.26%
<!--	Total:	17	28	60.71%	-->

Totals
Change from base Build 11600064225:	0.2%
Covered Lines:	1502
Relevant Lines:	2105

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💛 - Coveralls