Expose the Indexer publicly.

[poljar]

Some clients might already have an event store yet they might want to index events as well. One of those clients is for example Riot-X on Android.

Such clients will want to use only the indexing side of Seshat. Luckily the implementation already has a clear separation of the database and the indexer.

For this to be fixed the Index struct will need to be made public, documenting all the public methods will be necessary as well.