Upgrade to the 2018 edition

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

src/header/sys_socket/constants.rs

@@ -1,4 +1,4 @@
-use platform::types::*;
+use crate::platform::types::*;
 
 pub const SOCK_STREAM: c_int = 1;
 pub const SOCK_DGRAM: c_int = 2;

src/header/sys_socket/mod.rs

@@ -2,8 +2,8 @@
 
 use core::ptr;
 
-use platform::types::*;
-use platform::{PalSocket, Sys};
+use crate::platform::types::*;
+use crate::platform::{PalSocket, Sys};
 
 pub mod constants;