historical slashing w ocw w adhoc tree creation (#6220)

* draft

* steps

* chore: fmt

* step by step

* more details

* make test public

* refactor: split into on and offchain

* test stab

* tabs my friend

* offchain overlay: split key into prefix and true key

Simplifies inspection and makes key actually unique.

* test: share state

* fix & test

* docs improv

* address review comments

* cleanup test chore

* refactor, abbrev link text

* chore: linewidth

* fix prefix key split fallout

* minor fallout

* minor changes

* addresses review comments

* rename historical.rs -> historical/mod.rs

* avoid shared::* wildcard import

* fix: add missing call to store_session_validator_set_to_offchain

* fix/compile: missing shared:: prefix

* fix/test: flow

* fix/review: Apply suggestions from code review

Co-authored-by: Tomasz Drwięga <tomusdrw@users.noreply.github.com>

* fix/review: more review comment fixes

* fix/review: make ValidatorSet private

* fix/include: core -> sp_core

* fix/review: fallout

* fix/visbility: make them public API

Ref #6358

* fix/review: review changes fallout - again

Co-authored-by: Bernhard Schuster <bernhard@parity.io>
Co-authored-by: Tomasz Drwięga <tomusdrw@users.noreply.github.com>