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/context/timeout.rs

@@ -1,11 +1,11 @@
 use alloc::collections::VecDeque;
 use spin::{Once, Mutex, MutexGuard};
 
-use event;
-use scheme::SchemeId;
-use syscall::data::TimeSpec;
-use syscall::flag::{CLOCK_MONOTONIC, CLOCK_REALTIME, EVENT_READ};
-use time;
+use crate::event;
+use crate::scheme::SchemeId;
+use crate::syscall::data::TimeSpec;
+use crate::syscall::flag::{CLOCK_MONOTONIC, CLOCK_REALTIME, EVENT_READ};
+use crate::time;
 
 #[derive(Debug)]
 struct Timeout {