Merge branch 'fix-0.2.2' into 'master'

Add missing imports, fixing 0.2.2.

See merge request redox-os/syscall!60

Cargo.toml

 [package]
 name = "redox_syscall"
-version = "0.2.2"
+version = "0.2.3"
 description = "A Rust library to access raw Redox system calls"
 license = "MIT"
 authors = ["Jeremy Soller <jackpot51@gmail.com>"]

src/arch/x86_64.rs

+use core::{mem, slice};
+use core::ops::{Deref, DerefMut};
+
 use super::error::{Error, Result};
 
 macro_rules! syscall {