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/Object.h

@@ -189,6 +189,7 @@ public:
     virtual bool is_proxy_object() const { return false; }
     virtual bool is_native_function() const { return false; }
     virtual bool is_ecmascript_function_object() const { return false; }
+    virtual bool is_iterator_record() const { return false; }
 
     // B.3.7 The [[IsHTMLDDA]] Internal Slot, https://tc39.es/ecma262/#sec-IsHTMLDDA-internal-slot
     virtual bool is_htmldda() const { return false; }