LibJS: Add the SetIterator built-in and Set.prototype.{values, entries}

While this implementation should be complete it is based on HashTable's
iterator, which currently follows bucket-order instead of the required
insertion order. This can be simply fixed by replacing the underlying
HashTable member in Set with an enhanced one that maintains a linked
list in insertion order.

Userland/Libraries/LibJS/Runtime/SetIterator.cpp

@@ -0,0 +1,35 @@
+/*
+ * Copyright (c) 2021, Idan Horowitz <idan.horowitz@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibJS/Runtime/GlobalObject.h>
+#include <LibJS/Runtime/SetIterator.h>
+
+namespace JS {
+
+SetIterator* SetIterator::create(GlobalObject& global_object, Set& set, Object::PropertyKind iteration_kind)
+{
+    return global_object.heap().allocate<SetIterator>(global_object, *global_object.set_iterator_prototype(), set, iteration_kind);
+}
+
+SetIterator::SetIterator(Object& prototype, Set& set, Object::PropertyKind iteration_kind)
+    : Object(prototype)
+    , m_set(set)
+    , m_iteration_kind(iteration_kind)
+    , m_iterator(set.values().begin())
+{
+}
+
+SetIterator::~SetIterator()
+{
+}
+
+void SetIterator::visit_edges(Cell::Visitor& visitor)
+{
+    Base::visit_edges(visitor);
+    visitor.visit(&m_set);
+}
+
+}