Implement VarHandle invoker caching

Currently var handle invoker caching has some TODO comments:

    /*non-public*/
    MethodHandle varHandleMethodInvoker(VarHandle.AccessMode ak) {
        // TODO cache invoker
        return makeVarHandleMethodInvoker(ak, false);
    }

    /*non-public*/
    MethodHandle varHandleMethodExactInvoker(VarHandle.AccessMode ak) {
        // TODO cache invoker
        return makeVarHandleMethodInvoker(ak, true);
    }

It is relatively easy/simple to implement caching here, by replicating what is done for MethodHandle invokers (multiplied by the number of access modes)