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/Layer.h

@@ -14,7 +14,7 @@ private:
 	int64_t seed;
 
 protected:
-	shared_ptr<Layer>parent;
+	std::shared_ptr<Layer>parent;
 
 private:
 	int64_t rval;