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/Objects/Spinner.cs

@@ -10,5 +10,7 @@ namespace osu.Game.Modes.Osu.Objects
         public double Length;
 
         public override double EndTime => StartTime + Length;
+
+        public override HitObjectType Type => HitObjectType.Spinner;
     }
 }