Reformat Validator Election (#2406)

* Add index caching to election

* Initial draft of the new phragmen API.

* Port post-processing to the new API.

* Fix tests and accuracy.

* Final fixes.

* Unify convert namings.

* Remove todo.

* Some typos.

* Bump.

* Add extended balance type doc.

* A bit more sophisticated weight compensation.

* Fix review feedbacks.

* Bump.

* Final updates