Don't crash on printing nested types

When adding an Array or Dictionary to itself operator String() got in an
infinite loop. This commit adds a stack to operator String() (Through
the use of a new 'stringify method'). This stack keeps track of all
unique Arrays and Dictionaries it has seen. When a duplicate is found
only a static string is printed '[...]' or '{...}'.

This mirror Python's behavior in a similar case.

core/ordered_hash_map.h

@@ -274,6 +274,10 @@ public:
 	inline bool empty() const { return list.empty(); }
 	inline int size() const { return list.size(); }
 
+	const void *id() const {
+		return list.id();
+	}
+
 	void clear() {
 		map.clear();
 		list.clear();