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/value_iterate.wren

@@ -0,0 +1,45 @@
+var a = {"one": 1, "two": 2, "three": 3, "four": 4}.values
+
+// The precise numeric values aren't defined since they are indexes into the
+// entry table and the hashing process isn't specified. So we just validate
+// what we can assume about them.
+
+IO.print(a.iterate(null) is Num) // expect: true
+IO.print(a.iterate(null) >= 0) // expect: true
+
+IO.print(a.iterate(0) is Num) // expect: true
+IO.print(a.iterate(0) > 0) // expect: true
+IO.print(a.iterate(1) is Num) // expect: true
+IO.print(a.iterate(1) > 0) // expect: true
+IO.print(a.iterate(2) is Num) // expect: true
+IO.print(a.iterate(2) > 0) // expect: true
+IO.print(a.iterate(3) is Num) // expect: true
+IO.print(a.iterate(3) > 0) // expect: true
+
+var previous = -1
+var iterator = a.iterate(null)
+while (iterator) {
+  IO.print(iterator > previous)
+  IO.print(iterator is Num)
+  previous = iterator
+  iterator = a.iterate(iterator)
+}
+// First entry:
+// expect: true
+// expect: true
+// Second entry:
+// expect: true
+// expect: true
+// Third entry:
+// expect: true
+// expect: true
+// Fourth entry:
+// expect: true
+// expect: true
+
+// Out of bounds.
+IO.print(a.iterate(16)) // expect: false
+IO.print(a.iterate(-1)) // expect: false
+
+// Nothing to iterate in an empty map.
+IO.print({}.values.iterate(null)) // expect: false