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/memory/bump.rs

@@ -1,7 +1,7 @@
 //! # Bump frame allocator
 //! Some code was borrowed from [Phil Opp's Blog](http://os.phil-opp.com/allocating-frames.html)
 
-use paging::PhysicalAddress;
+use crate::paging::PhysicalAddress;
 
 use super::{Frame, FrameAllocator, MemoryArea, MemoryAreaIter};
 

src/memory/mod.rs

@@ -1,7 +1,7 @@
 //! # Memory management
 //! Some code was borrowed from [Phil Opp's Blog](http://os.phil-opp.com/allocating-frames.html)
 
-pub use paging::{PAGE_SIZE, PhysicalAddress};
+pub use crate::paging::{PAGE_SIZE, PhysicalAddress};
 
 use self::bump::BumpAllocator;
 use self::recycle::RecycleAllocator;

src/memory/recycle.rs

@@ -3,7 +3,7 @@
 
 use alloc::vec::Vec;
 
-use paging::PhysicalAddress;
+use crate::paging::PhysicalAddress;
 
 use super::{Frame, FrameAllocator};