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/Forward.h

@@ -187,7 +187,7 @@ struct ImportEntry;
 class ImportStatement;
 class Identifier;
 class Intrinsics;
-struct IteratorRecord;
+class IteratorRecord;
 class MemberExpression;
 class MetaProperty;
 class Module;