Use new Unique API (#346)

Change Unique to use `new_unchecked`. Fixes #345.

blog/content/posts/06-page-tables/index.md

@@ -659,7 +659,7 @@ Because the `ActivePageTable` owns the unique recursive mapped P4 table, there m
 impl ActivePageTable {
     pub unsafe fn new() -> ActivePageTable {
         ActivePageTable {
-            p4: Unique::new(table::P4),
+            p4: Unique::new_unchecked(table::P4),
         }
     }
 }