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/syscall/validate.rs

@@ -1,8 +1,8 @@
 use core::{mem, slice};
 
-use paging::{ActivePageTable, Page, VirtualAddress};
-use paging::entry::EntryFlags;
-use syscall::error::*;
+use crate::paging::{ActivePageTable, Page, VirtualAddress};
+use crate::paging::entry::EntryFlags;
+use crate::syscall::error::*;
 
 fn validate(address: usize, size: usize, flags: EntryFlags) -> Result<()> {
     let end_offset = size.checked_sub(1).ok_or(Error::new(EFAULT))?;