echo-server: Update.

echo-server/Cargo.toml

@@ -2,5 +2,6 @@
 name = "echo-server"
 version = "0.1.0"
 authors = ["You <you@example.com>"]
+edition = "2018"
 
 [dependencies]

echo-server/src/main.rs

@@ -1,3 +1,6 @@
+#![warn(rust_2018_idioms)]
+#![allow(elided_lifetimes_in_paths)]
+
 use std::net::TcpListener;
 use std::io;
 use std::thread::spawn;