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/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]}