Migrate import statements and macros to Rust 2018.

examples/crypto_bench.rs

@@ -14,12 +14,10 @@
 
 #![no_std]
 
-#[macro_use]
 extern crate alloc;
-extern crate crypto;
 extern crate lang_items;
-extern crate libtock_drivers;
 
+use alloc::format;
 use alloc::vec::Vec;
 use core::fmt::Write;
 use crypto::{

examples/oom_test.rs

@@ -15,9 +15,7 @@
 #![no_std]
 
 extern crate alloc;
-extern crate crypto;
 extern crate lang_items;
-extern crate libtock_drivers;
 
 use alloc::vec::Vec;
 use core::fmt::Write;

examples/panic_test.rs

@@ -14,10 +14,7 @@
 
 #![no_std]
 
-extern crate alloc;
-extern crate crypto;
 extern crate lang_items;
-extern crate libtock_drivers;
 
 fn main() {
     panic!("Bye world!")