Key and value iteration for maps. Also toString.

test/map/to_string.wren

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+// Handle empty map.
+IO.print({}.toString)             // expect: {}
+
+// Does not quote strings.
+IO.print({"1": "2"}.toString) // expect: {1: 2}
+
+// Nested maps.
+IO.print({1: {2: {}}}) // expect: {1: {2: {}}}
+
+// Calls toString on elements.
+class Foo {
+  toString { "Foo.toString" }
+}
+
+IO.print({1: new Foo}) // expect: {1: Foo.toString}
+
+// Since iteration order is unspecified, we don't know what order the results
+// will be.
+var s = {1: 2, 3: 4, 5: 6}.toString
+IO.print(s == "{1: 2, 3: 4, 5: 6}" ||
+         s == "{1: 2, 5: 6, 3: 4}" ||
+         s == "{3: 4, 1: 2, 5: 6}" ||
+         s == "{3: 4, 5: 6, 1: 2}" ||
+         s == "{5: 6, 1: 2, 3: 4}" ||
+         s == "{5: 6, 3: 4, 1: 2}") // expect: true
+
+// TODO: Handle maps that contain themselves.