title: RxJava2 could not find Func0
body: <p>RxJava 2 almost released and I want to migrate from RX 1.2.0 to 2.0.0, But I have noticed that there is no Func0 interface in RxJava 2.</p>
  
  <p>What developers should use instead of Func0 in RxJava 2? </p>