Don't stackoverflow on recursive lists and maps. Fix #3.

test/core/list/to_string.wren

@@ -14,4 +14,37 @@ class Foo {
 
 IO.print([1, new Foo, 2]) // expect: [1, Foo.toString, 2]
 
-// TODO: Handle lists that contain themselves.
+// Lists that directly contain themselves.
+var list = []
+list.add(list)
+IO.print(list) // expect: [...]
+
+list = [1, 2]
+list[0] = list
+IO.print(list) // expect: [..., 2]
+
+list = [1, 2]
+list[1] = list
+IO.print(list) // expect: [1, ...]
+
+// Lists that indirectly contain themselves.
+list = [null, [2, [3, null, 4], null, 5], 6]
+list[0] = list
+list[1][1][1] = list
+list[1][2] = list
+IO.print(list) // expect: [..., [2, [3, ..., 4], ..., 5], 6]
+
+// List containing an object that calls toString on a recursive list.
+class Box {
+  new(field) { _field = field }
+  toString { "box " + _field.toString }
+}
+
+list = [1, 2]
+list.add(new Box(list))
+IO.print(list) // expect: [1, 2, box ...]
+
+// List containing a map containing the list.
+list = [1, null, 2]
+list[1] = {"list": list}
+IO.print(list) // expect: [1, {list: ...}, 2]