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/Snowball.h

@@ -16,7 +16,7 @@ private:
 
 public:
 	Snowball(Level *level);
-	Snowball(Level *level, shared_ptr<Mob> mob);
+	Snowball(Level *level, std::shared_ptr<Mob> mob);
 	Snowball(Level *level, double x, double y, double z);
 
 protected: