Upgrade to the 2018 edition

I didn't think it'd be this useful first, but thank god for `cargo fix --edition`!

src/header/float/mod.rs

@@ -1,8 +1,8 @@
 //! float.h implementation for Redox, following
 //! http://pubs.opengroup.org/onlinepubs/7908799/xsh/float.h.html
 
-use header::_fenv::{fegetround, FE_TONEAREST};
-use platform::types::*;
+use crate::header::_fenv::{fegetround, FE_TONEAREST};
+use crate::platform::types::*;
 
 pub const FLT_RADIX: c_int = 2;