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/array.cpp

@@ -401,6 +401,10 @@ Variant Array::max() const {
 	return maxval;
 }
 
+const void *Array::id() const {
+	return _p->array.ptr();
+}
+
 Array::Array(const Array &p_from) {
 
 	_p = NULL;