Made GFS code into an ADT

src/gtfs/utils.rs

@@ -1,6 +1,5 @@
 use std::collections::HashSet;
-use crate::gtfs::structs::Gtfs;
-
+use crate::gtfs::Gtfs;
 
 pub fn stop_ids_from_codes(gtfs: &Gtfs, stop_codes: &HashSet<String>) -> HashSet<String> {
     let mut ids: HashSet<String> = HashSet::new();