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/tests/mod.rs

@@ -24,6 +24,6 @@ fn stdio() {
 /// Test that invalid reads/writes cause errors
 #[test]
 fn invalid_path() {
-    assert_eq!(syscall::read(999, &mut []), Err(Error::new(EBADF)));
-    assert_eq!(syscall::write(999, &[]), Err(Error::new(EBADF)));
+    assert_eq!(syscall::read(999, &mut []), Err(Error::new(syscall::EBADF)));
+    assert_eq!(syscall::write(999, &[]), Err(Error::new(syscall::EBADF)));
 }