Rename `fin_of_nat` → `nat_to_fin` to follow the conventions.
Rename fin_of_nat
into nat_to_fin
, fin_to_of_nat
into fin_to_nat_to_fin
, and fin_of_to_nat
into nat_to_fin_to_nat
, to follow the conventions in Iris, and also the recent rename for the similar vec
functions.