Implement proper, efficient persistent data structures

[Tko1]

Most likely this will just involve integrating https://docs.rs/im/14.3.0/im/ into

persistent_vector.rs

and adding a persistent_hashmap.rs also built on this (not to be confused with our already existing persistent_list_map, which is a non-efficient persistent hashmap implemented as an associative-list for small datasets and, currently, used for prototyping things related to maps)