message: chat_example adapt to serde
diff: -use moonlight::serde_lite::{self, Deserialize, Serialize};
  +use moonlight::*;
  // ------ UpMsg ------
  #[derive(Serialize, Deserialize, Debug)]
  +#[serde(crate = "serde")]
  pub enum UpMsg {
  SendMessage(Message),
  }
  @@ -10,6 +11,7 @@ pub enum UpMsg {
  // ------ DownMsg ------
  #[derive(Serialize, Deserialize, Debug)]
  +#[serde(crate = "serde")]
  pub enum DownMsg {
  MessageReceived(Message),
  }
  @@ -17,6 +19,7 @@ pub enum DownMsg {
  // ------ Message ------
  #[derive(Serialize, Deserialize, Clone, Debug)]
  +#[serde(crate = "serde")]
  pub struct Message {
  pub username: String,
  pub text: String,
  