chore: fix ci failure on master (#2593)

* Fix clippy warnings
* Pin rustc version to 1.43.1 in macOS

Refs: https://github.com/rust-lang/rust/issues/73030

examples/echo-udp.rs

@@ -15,7 +15,6 @@
 use std::error::Error;
 use std::net::SocketAddr;
 use std::{env, io};
-use tokio;
 use tokio::net::UdpSocket;
 
 struct Server {

examples/echo.rs

@@ -21,7 +21,6 @@
 
 #![warn(rust_2018_idioms)]
 
-use tokio;
 use tokio::io::{AsyncReadExt, AsyncWriteExt};
 use tokio::net::TcpListener;
 

examples/tinyhttp.rs

@@ -18,7 +18,6 @@ use futures::SinkExt;
 use http::{header::HeaderValue, Request, Response, StatusCode};
 #[macro_use]
 extern crate serde_derive;
-use serde_json;
 use std::{env, error::Error, fmt, io};
 use tokio::net::{TcpListener, TcpStream};
 use tokio::stream::StreamExt;