feat: allow multi fastas

[JeremyWesthead]

Enables parsing FN6 saves from (gzipped) multi fastas which should improve use experience

[mcolpus]

Looks good to me from reading through code changes. Just to confirm your intending to remove the tree building package (I assume because we don't really use it)

[JeremyWesthead]

It's a slightly niche use case, but actually the reason for removal is difficulties with the python packaging. It would either package just the scripts or just the rust library. Obviously the priority is the rust library, and without the packaging, they become relatively useless in this repo

I have vague (incredibly low priority) ideas about adding a rust implementation to keep the functionality as it's quite neat though