Refactor medschool to use a simple DFS

[infiniteregrets]

A few important things here:

• The earlier medschool implementation used a BtreeSet which was not required as we could simple depth first search on a vector of types with a hashmap based lookup. I intend to optimize this further so this is WIP.
• Files are read in parallel and parsed with a threadpool, but we need to benchmark if rayon.par_into_iter is helpful without the bloat.
• Some other improvements like squashing multiple filter_maps into a single.

WIP..