fix: prevent stack overflow on recursive toString for List and Map

Add `String.safeToString_` static method that tracks objects currently being
converted to string per fiber, returning "..." when recursion is detected.
Wrap List.toString and Map.toString with this guard to handle self-referential
and mutually recursive structures without crashing.

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]

test/core/map/to_string.wren

@@ -24,4 +24,28 @@ IO.print(s == "{1: 2, 3: 4, 5: 6}" ||
          s == "{5: 6, 1: 2, 3: 4}" ||
          s == "{5: 6, 3: 4, 1: 2}") // expect: true
 
-// TODO: Handle maps that contain themselves.
+// Map that directly contains itself.
+var map = {}
+map["key"] = map
+IO.print(map) // expect: {key: ...}
+
+// Map that indirectly contains itself.
+map = {}
+map["a"] = {"b": {"c": map}}
+
+IO.print(map) // expect: {a: {b: {c: ...}}}
+
+// Map containing an object that calls toString on a recursive map.
+class Box {
+  new(field) { _field = field }
+  toString { "box " + _field.toString }
+}
+
+map = {}
+map["box"] = new Box(map)
+IO.print(map) // expect: {box: box ...}
+
+// Map containing a list containing the map.
+map = {}
+map["list"] = [1, map, 2]
+IO.print(map) // expect: {list: [1, ..., 2]}