fix "extern crate core" import

Broken on latest rust nightly because using no_std automatically injects
an "extern crate core". (rust-lang/rfcs#1184)

Author: joelarmstrong

src/lib.rs

@@ -13,13 +13,10 @@
 #![crate_type="lib"]
 
 #![feature(asm, no_std)]
-#![feature(core)]
 #![deny(warnings)]
 #![no_std]
 #![allow(unused_mut)]
 
-extern crate core;
-
 #[cfg(test)]
 extern crate std;