Implement a rank fusion index

[DrDub]

The current RerankedIndex will completely miss documents that the main search engine cannot see (e.g., if the main search engine is keyword based, a document with no keyword overlap will go unseen; if the main search engine is embedding based, a document with a out-of-vocabulary term will go unseen, even with query overlap).

Instead of reranking, it'd be advantageous to implement Reciprocal Rank Fusion (RRF) from Cormack et al., SIGIR'09:

https://plg.uwaterloo.ca/~gvcormac/cormacksigir09-rrf.pdf