Fixes for rust nightly change to behavior of no_std[1]

[1]rust-lang/rfcs#1184

Author: mokus0

src/lib.rs

@@ -21,14 +21,12 @@
 //! the system libc library.
 
 #![no_std]
-#![feature(no_std, core)]
+#![feature(no_std)]
 
 // This library defines the builtin functions, so it would be a shame for
 // LLVM to optimize these function calls to themselves!
 #![no_builtins]
 
-extern crate core;
-
 #[cfg(test)] #[macro_use] extern crate std;
 
 #[no_mangle]