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/AddSnowLayer.h

@@ -4,6 +4,6 @@
 class AddSnowLayer : public Layer
 {
 public:
-    AddSnowLayer(int64_t seedMixup, shared_ptr<Layer> parent);
+    AddSnowLayer(int64_t seedMixup, std::shared_ptr<Layer> parent);
     virtual intArray getArea(int xo, int yo, int w, int h);
 };