Introduce srml/scored-pool (#3381)

* Introduce srml/scored-pool

* Bump impl_version

* Apply suggestions from code review

Co-Authored-By: Bastian Köcher <bkchr@users.noreply.github.com>

* Remove unnecessary pub use

* Remove unnecessary import

* Adapt to InitializeMembers

* Bump impl_version

* Implement remarks (shortens code)

* Improve complexity of score()

Search and remove and search again for the
new spot and insert then => O(2LogN).

* Get rid of a clone()

* Reduce complexity of issue_candidacy()

* Add CandidateScored event + Improve comments

* Fix naming

* Use Lookup instead of AccountId as param

* Use set_members_sorted instead of computing diff

* Remove function which is only used during genesis

* Get rid of rev() by changing sort order of Pool

* Rename issue_candidacy to submit_candidacy

* Shorten code

* Remove find_in_pool() and have transactor submit index

* Remove unnecessary dependency

* Improve error messages

* Improve naming

* Improve comments

* Make code clearer wrt which receiver to invoke

* Adapt to new system trait

* Refactor to request CandidateDeposit only once

* Refactor to request Pool only once

* Improve structure and comments