LibJS: Stop converting between Object <-> IteratorRecord all the time

This patch makes IteratorRecord an Object. Although it's not exposed to
author code, this does allow us to store it in a VM register.

Now that we can store it in a VM register, we don't need to convert it
back and forth between IteratorRecord and Object when accessing it from
bytecode.

The big win here is avoiding 3 [[Get]] accesses on every iteration step
of for..of loops. There are also a bunch of smaller efficiencies gained.

20% speed-up on this microbenchmark:

    function go(a) {
        for (const p of a) {
        }
    }
    const a = [];
    a.length = 1_000_000;
    go(a);

Userland/Libraries/LibJS/Runtime/IteratorConstructor.cpp

@@ -70,12 +70,12 @@ JS_DEFINE_NATIVE_FUNCTION(IteratorConstructor::from)
     auto iterator_record = TRY(get_iterator_flattenable(vm, object, StringHandling::IterateStrings));
 
     // 2. Let hasInstance be ? OrdinaryHasInstance(%Iterator%, iteratorRecord.[[Iterator]]).
-    auto has_instance = TRY(ordinary_has_instance(vm, iterator_record.iterator, realm.intrinsics().iterator_constructor()));
+    auto has_instance = TRY(ordinary_has_instance(vm, iterator_record->iterator, realm.intrinsics().iterator_constructor()));
 
     // 3. If hasInstance is true, then
     if (has_instance.is_boolean() && has_instance.as_bool()) {
         // a. Return iteratorRecord.[[Iterator]].
-        return iterator_record.iterator;
+        return iterator_record->iterator;
     }
 
     // 4. Let wrapper be OrdinaryObjectCreate(%WrapForValidIteratorPrototype%, « [[Iterated]] »).