Cleanup of the `state-machine` crate (#3524)

* Start refactoring state-machine crate

* More improvement to state-machine

* Fix tests compilation on master and remove warnings

* Fix compilation

* Apply suggestions from code review

Co-Authored-By: Sergei Pepyakin <sergei@parity.io>

* Update core/state-machine/src/basic.rs

Co-Authored-By: DemiMarie-parity <48690212+DemiMarie-parity@users.noreply.github.com>

* Line width

* Update core/primitives/src/storage.rs

Co-Authored-By: Benjamin Kampmann <ben.kampmann@googlemail.com>

* Update core/state-machine/src/error.rs

Co-Authored-By: Benjamin Kampmann <ben.kampmann@googlemail.com>

* Review feedback