HitObjects are not guaranteed to be ordered

Hit objects need to be sorted by initial timestamp. Osu format does not require them to be in order by time and as such, maps exist with unordered notes. Example: https://osu.ppy.sh/beatmapsets/724477#mania/1530525 (towards the end, a handful of notes from the beginning were inserted). Other functions that iterate through the list of notes assume the notes are ordered.