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/VM.cpp

@@ -226,7 +226,7 @@ ThrowCompletionOr<void> VM::binding_initialization(NonnullRefPtr<BindingPattern
         auto result = iterator_binding_initialization(*target, iterator_record, environment);
 
         // 3. If iteratorRecord.[[Done]] is false, return ? IteratorClose(iteratorRecord, result).
-        if (!iterator_record.done) {
+        if (!iterator_record->done) {
             // iterator_close() always returns a Completion, which ThrowCompletionOr will interpret as a throw
             // completion. So only return the result of iterator_close() if it is indeed a throw completion.
             auto completion = result.is_throw_completion() ? result.release_error() : normal_completion({});