* guide: validation data refactoring

* address grumbles from review

* Update roadmap/implementers-guide/src/types/candidate.md

Co-authored-by: Bernhard Schuster <bernhard@ahoi.io>

* last comments from review

Co-authored-by: Sergei Shulepov <sergei@parity.io>
Co-authored-by: Bernhard Schuster <bernhard@ahoi.io>