Update Tokio to Rust 2018 (#1082)

tokio-threadpool/tests/blocking.rs

@@ -1,20 +1,14 @@
-extern crate tokio_threadpool;
-
-extern crate env_logger;
-extern crate futures;
-extern crate rand;
-
-use tokio_threadpool::*;
+#![deny(warnings, rust_2018_idioms)]
 
 use futures::future::{lazy, poll_fn};
 use futures::*;
 use rand::*;
-
 use std::sync::atomic::Ordering::*;
 use std::sync::atomic::*;
 use std::sync::*;
 use std::thread;
 use std::time::Duration;
+use tokio_threadpool::*;
 
 #[test]
 fn basic() {