Add ChangeToken static methods to Primitives

[rynowak]

We want to add these boilerplate combinators for IChangeToken

public static class ChangeToken
{
    public static void OnChange(Func<IChangeToken> changeTokenProducer, Action changeTokenConsumer)
    {
        Action<object> callback = null;
        callback = (s) => 
        {
            var token = changeTokenProducer();
            changeTokenConsumer();
            token.Register(callback, null);
        }

        token.Register(callback, null);
    }

    public static void OnChange<T>(Func<IChangeToken> changeTokenProducer, Action<T> changeTokenConsumer, T state)
    {
        Action<object> callback = null;
        callback = (s) => 
        {
            var token = changeTokenProducer();
            changeTokenConsumer((T)s);
            token.Register(callback, s);
        }

        token.Register(callback, state);
    }
}

[HaoK]

01eff58