ethcore: fix pow difficulty validation (#9328)

* ethcore: fix pow difficulty validation

* ethcore: validate difficulty is not zero

* ethcore: add issue link to regression test

* ethcore: fix tests

* ethcore: move difficulty_to_boundary to ethash crate

* ethcore: reuse difficulty_to_boundary and boundary_to_difficulty

* ethcore: fix grumbles in difficulty_to_boundary_aux