bugfix in buffer object. Didn't increase the refcount when slicing

examples/data_structures/stack.noja

@@ -1,67 +0,0 @@
-# Implementation of a stack using a linked list.
-
-fun Stack_New(T) {
-	if T == None:
-		return none, "The type can't be None";
-	return { count: 0, head: none, type: T };
-}
-
-fun Stack_Push(stack: Map, value) {
-	
-	assert(stack.type != None);
-
-	if type(value) != stack.type:
-		error("Invalid type");
-
-	node = {prev: none, item: value};
-
-	node.prev = stack.head;
-	stack.head = node;
-	stack.count = stack.count + 1;
-}
-
-fun Stack_Pop(stack: Map) {
-
-	if stack.head == none:
-		return none;
-	
-	value = stack.head.item;
-	stack.head = stack.head.prev;
-	stack.count = stack.count - 1;
-
-	return value;
-}
-
-fun Stack_Print(stack: Map) {
-	print("[");
-	curr = stack.head;
-	while curr != none: {
-		print(curr.item);
-		curr = curr.prev;
-		if curr != none:
-			print(", ");
-	}
-	print("]");
-}
-
-stack = Stack_New(int);
-
-Stack_Push(stack, 1);
-Stack_Push(stack, 2);
-Stack_Push(stack, 3);
-
-Stack_Print(stack);
-print("\n");
-
-x = Stack_Pop(stack);
-y = Stack_Pop(stack);
-z = Stack_Pop(stack);
-w = Stack_Pop(stack);
-
-assert(x == 3);
-assert(y == 2);
-assert(z == 1);
-assert(w == none);
-
-Stack_Print(stack);
-print("\n");