BABE refactoring: split out verification (#3658)

* Refactor parts of BABE verification into separate module

* Fix silly compiler error

* Move more of the verification code to verification.rs

* Remove some unused imports

* Fix line width

* fix testsuite compile error

* Fix compile errors in tests

* Move authorship-related code to its own files

* fix compile errors in tests

* Respond to review comments by @rphmeier

* improve docs

* fix compile error

* Add missing doc comment