shared_ptr -> std::shared_ptr

This is one of the first commits in a plan to remove all `using namespace std;` lines in the entire codebase as it is considered anti-pattern today.

Minecraft.World/HellSandTile.h

@@ -7,5 +7,5 @@ class HellSandTile : public Tile
 public:
 	HellSandTile(int id);
 	virtual AABB *getAABB(Level *level, int x, int y, int z);
-	virtual void entityInside(Level *level, int x, int y, int z, shared_ptr<Entity> entity);
+	virtual void entityInside(Level *level, int x, int y, int z, std::shared_ptr<Entity> entity);
 };