Revert 40897f3348.

It leaks memory in the case of runtime errors.

test/core/map/to_string.wren

@@ -24,28 +24,4 @@ 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
 
-// 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]}
+// TODO: Handle maps that contain themselves.