Improve data structures

[georgematheos]

Currently, there are several ways the data structures for the world could be improved:

1. Several data structures could probably be streamlined by using some more general-purpose data structure. Perhaps a PersistentAddressTree/PersistentTrie would be useful. MultiSet is probably also useful (Use multiset for lookup count tracking #34).
2. I currently use NamedTuples instead of PersistentHashMaps in several data structures; I had thought when I implemented this that this would be more performant, but I'm not very confident about this. It is probably worth checking
3. Using https://github.com/RelationalAI-oss/MuttsDicts.jl instead of FunctionalCollections may yield performance benefits.