Add difficulty calculation

Adds base classes for difficulty calculations, hooks them up with
carousel container, and adds a port of the osu difficulty calculator.

osu.Game.Modes.Osu/OsuRuleset.cs

@@ -40,6 +40,8 @@ namespace osu.Game.Modes.Osu
 
         public override ScoreProcessor CreateScoreProcessor(int hitObjectCount) => new OsuScoreProcessor(hitObjectCount);
 
+        public override DifficultyCalculator CreateDifficultyCalculator(Beatmap beatmap) => new OsuDifficultyCalculator(beatmap);
+
         protected override PlayMode PlayMode => PlayMode.Osu;
     }
 }