Rearranged the existing code to make it more manageable.

src/gtfs/structs.rs

@@ -0,0 +1,22 @@
+use std::collections::HashMap;
+
+use gtfs_structures::{Agency, Calendar, CalendarDate, RawStopTime, RawTrip, Route, Stop};
+
+// The main GTFS struct. This is similar to (but not exactly) gtfs-structures::Gtfs because we don't need everything
+#[derive(Debug)]
+pub struct Gtfs {
+    /// All agencies. They can not be read by `agency_id`, as it is not a required field
+    pub agencies: Vec<Agency>,
+    /// All Calendar by `service_id`
+    pub calendar: HashMap<String, Calendar>,
+    /// All calendar dates grouped by service_id
+    pub calendar_dates: HashMap<String, Vec<CalendarDate>>,
+    /// All routes by `route_id`
+    pub routes: HashMap<String, Route>,
+    /// All stop by `stop_id`.
+    pub stops: HashMap<String, Stop>,
+    /// All trips by trip_id
+    pub trips: HashMap<String, RawTrip>,
+    /// Stop times for the chosen stops and the chosen routes
+    pub stop_times: HashMap<(String, u32), RawStopTime>,
+}