Switch to 2018 edition

Most of this was generated by the absolutely extraordinary `cargo fix`
subcommand. There were still 2 errors and a few warnings to patch up,
but compared to the normal 600+ errors, I'd say the fixer did a damn
good job! I'm also amazed that I could still start the VM after this,
I half expected some kinds of runtime failure...

src/sync/wait_condition.rs

@@ -2,7 +2,7 @@ use alloc::sync::Arc;
 use alloc::vec::Vec;
 use spin::{Mutex, RwLock};
 
-use context::{self, Context};
+use crate::context::{self, Context};
 
 #[derive(Debug)]
 pub struct WaitCondition {

src/sync/wait_map.rs

@@ -2,7 +2,7 @@ use alloc::collections::BTreeMap;
 use core::mem;
 use spin::Mutex;
 
-use sync::WaitCondition;
+use crate::sync::WaitCondition;
 
 #[derive(Debug)]
 pub struct WaitMap<K, V> {

src/sync/wait_queue.rs

@@ -1,7 +1,7 @@
 use alloc::collections::VecDeque;
 use spin::Mutex;
 
-use sync::WaitCondition;
+use crate::sync::WaitCondition;
 
 #[derive(Debug)]
 pub struct WaitQueue<T> {