feat: add first hard example

Hard example tests all actual language features. For now its:
- Classes
- Methods
- Recursive calls
- Object move and borrowing

exl/hard-1.fzs

@@ -0,0 +1,61 @@
+class Node
+{
+  int value;
+  Node next;
+
+  fn append(int val) Node
+  {
+    if (this.next == null)
+    {
+      Node tail = new Node;
+      tail.value = val;
+      tail.next = null;
+      this.next = tail;
+      return this;
+    }
+
+    this.next = this.next.append(val);
+    return this;
+  }
+
+  fn sum() int
+  {
+    if (this.next == null)
+      return this.value;
+
+    return this.value + this.next.sum();
+  }
+
+  fn printList()
+  {
+    printline(this.value);
+    if (this.next != null)
+      this.next.printList();
+  }
+}
+
+fn createList(int count) Node
+{
+  if (count <= 0)
+    return null;
+
+  Node head = new Node;
+  head.value = count;
+  head.next = createList(count - 1);
+  return head;
+}
+
+printline("===== Test 1: factory & recursive escaping =====");
+Node list = createList(3);
+printline("Elements:");
+list.printList();
+
+printline("===== Test 2: mutation & methods =====");
+int total = list.sum();
+printline("Sum of elements:");
+printline(total);
+
+if (total == 16)
+  printline("Tests passed!");
+else printline("Tests failed");
+