* Sketch of indexed events.
* Get EventIndex by holding another variable.
* Add some docs.
* Use DoubleMap to store reverse topic index
* Implement StorageDoubleMap::append
* Use append for EventTopics.
* Refactor.
* Avoid `mutate`
* Docs.
* Add topics to EventRecord
* Update tests.
* Rebuild.
* Bump version.
* Event topics test.
* Mix in BlockNumber to distinguish updates
* Fix srml-system test.
* Post merge fixes.
* Comments/TODO.