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/Bytecode/Instruction.h

@@ -46,8 +46,10 @@
     O(GetByValueWithThis)              \
     O(GetCalleeAndThisFromEnvironment) \
     O(GetIterator)                     \
+    O(GetObjectFromIteratorRecord)     \
     O(GetMethod)                       \
     O(GetNewTarget)                    \
+    O(GetNextMethodFromIteratorRecord) \
     O(GetImportMeta)                   \
     O(GetObjectPropertyIterator)       \
     O(GetPrivateById)                  \