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.

builtin/core.wren

@@ -89,8 +89,39 @@ class List is Sequence {
 }
 
 class Map {
-  // TODO: Implement this.
-  toString { "{}" }
+  keys { new MapKeySequence(this) }
+  values { new MapValueSequence(this) }
+
+  toString {
+    var first = true
+    var result = "{"
+
+    for (key in keys) {
+      if (!first) result = result + ", "
+      first = false
+      result = result + key.toString + ": " + this[key].toString
+    }
+
+    return result + "}"
+  }
+}
+
+class MapKeySequence is Sequence {
+  new(map) {
+    _map = map
+  }
+
+  iterate(n) { _map.iterate_(n) }
+  iteratorValue(iterator) { _map.keyIteratorValue_(iterator) }
+}
+
+class MapValueSequence is Sequence {
+  new(map) {
+    _map = map
+  }
+
+  iterate(n) { _map.iterate_(n) }
+  iteratorValue(iterator) { _map.valueIteratorValue_(iterator) }
 }
 
 class Range is Sequence {}