* Introduce `CollectCollationInfo` runtime api

Instead of using well known keys to communicate information about a
collation between the runtime and the collator, we now use a runtime api
for this.

* Fixes bug

* Apply suggestions from code review

Co-authored-by: Sergei Shulepov <sergei@parity.io>

* Doc update

Co-authored-by: Sergei Shulepov <sergei@parity.io>