style(lib): use rust 2018 edition idioms (#1910)

examples/client.rs

@@ -1,7 +1,5 @@
 #![deny(warnings)]
-extern crate hyper;
-extern crate pretty_env_logger;
-
+#![warn(rust_2018_idioms)]
 use std::env;
 use std::io::{self, Write};
 

examples/client_json.rs

@@ -1,9 +1,8 @@
 #![deny(warnings)]
-extern crate hyper;
+#![warn(rust_2018_idioms)]
+
 #[macro_use]
 extern crate serde_derive;
-extern crate serde;
-extern crate serde_json;
 
 use hyper::Client;
 use futures_util::TryStreamExt;

examples/multi_server.rs

@@ -1,6 +1,5 @@
 #![deny(warnings)]
-extern crate hyper;
-extern crate pretty_env_logger;
+#![warn(rust_2018_idioms)]
 
 use hyper::{Body, Request, Response, Server};
 use hyper::service::{service_fn, make_service_fn};

examples/params.rs

@@ -1,7 +1,5 @@
 // #![deny(warnings)]  // FIXME: https://github.com/rust-lang/rust/issues/62411
-extern crate hyper;
-extern crate pretty_env_logger;
-extern crate url;
+#![warn(rust_2018_idioms)]
 
 use hyper::{Body, Method, Request, Response, Server, StatusCode};
 use hyper::service::{service_fn, make_service_fn};