Remove core_intrinsics feature

Stable versions of these functions now exist under core::ptr, so use
those.

src/lib.rs

@@ -1,7 +1,6 @@
 #![deny(warnings)]
 #![feature(asm)]
 #![feature(const_fn)]
-#![feature(core_intrinsics)]
 #![no_std]
 
 pub use self::arch::*;