feat: implement Map.keys, Map.values iterators and proper toString with key-value formatting

Add MapKeySequence and MapValueSequence classes in core.wren to expose iterable key and value views on Map instances. Implement native iterate_, keyIteratorValue_, and valueIteratorValue_ primitives in wren_core.c to support these sequences. Replace the stub Map.toString with a full implementation that iterates keys and formats each entry as "key: value", handling empty maps and nested maps correctly. Include comprehensive test suites for key iteration, value iteration, iterator type validation, and toString output across multiple orderings.

test/map/to_string.wren

@@ -0,0 +1,27 @@
+// 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.