Offences report system rework (#13425)

* Experiments with common equivocation trait

* Improved equivocation trait

* Fix grandpa equivocation implementation

* Remove some cruft

* Remove some more cruft

* More generic naming

* Simplification of offences manipilation

* More refactory

* Some prograss with the encapsulation of offence report system

* Finally unit type works as a universal null report system

* Align substrate node code

* Further simplification

* Fix test utils

* Remove not required associated type

* Fix benches

* Rollback to prev field name

* Box big params

* Fix typo

* Remove new tag computation

* Remove default implementations

* Better docs

* Return 'Result' instead of bool

* Change offence report system return types

* Some renaming and documentation

* Improve documentation

* More abstract offence report system

* Rename 'consume_evidence' to 'process_evidence'

* Further docs refinements

* Doc for dummy offence report

* Fix rustdoc

* Fix after master merge

* Apply code review suggestions

* Improve docs