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/allocator/linked_list.rs

@@ -3,7 +3,7 @@ use core::ptr::NonNull;
 use linked_list_allocator::Heap;
 use spin::Mutex;
 
-use paging::ActivePageTable;
+use crate::paging::ActivePageTable;
 
 static HEAP: Mutex<Option<Heap>> = Mutex::new(None);
 
@@ -32,10 +32,10 @@ unsafe impl GlobalAlloc for Allocator {
                         panic!("__rust_allocate: heap not initialized");
                     };
 
-                    super::map_heap(&mut ActivePageTable::new(), ::KERNEL_HEAP_OFFSET + size, ::KERNEL_HEAP_SIZE);
+                    super::map_heap(&mut ActivePageTable::new(), crate::KERNEL_HEAP_OFFSET + size, crate::KERNEL_HEAP_SIZE);
 
                     if let Some(ref mut heap) = *HEAP.lock() {
-                        heap.extend(::KERNEL_HEAP_SIZE);
+                        heap.extend(crate::KERNEL_HEAP_SIZE);
                     } else {
                         panic!("__rust_allocate: heap not initialized");
                     }

src/allocator/mod.rs

@@ -1,6 +1,6 @@
-use paging::{ActivePageTable, Page, VirtualAddress};
-use paging::entry::EntryFlags;
-use paging::mapper::MapperFlushAll;
+use crate::paging::{ActivePageTable, Page, VirtualAddress};
+use crate::paging::entry::EntryFlags;
+use crate::paging::mapper::MapperFlushAll;
 
 #[cfg(not(feature="slab"))]
 pub use self::linked_list::Allocator;
@@ -28,8 +28,8 @@ unsafe fn map_heap(active_table: &mut ActivePageTable, offset: usize, size: usiz
 }
 
 pub unsafe fn init(active_table: &mut ActivePageTable) {
-    let offset = ::KERNEL_HEAP_OFFSET;
-    let size = ::KERNEL_HEAP_SIZE;
+    let offset = crate::KERNEL_HEAP_OFFSET;
+    let size = crate::KERNEL_HEAP_SIZE;
 
     // Map heap pages
     map_heap(active_table, offset, size);