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/Slider.cs

@@ -110,6 +110,8 @@ namespace osu.Game.Modes.Osu.Objects
                 }
             }
         }
+
+        public override HitObjectType Type => HitObjectType.Slider;
     }
 
     public enum CurveTypes