Move Frame::start_address to memory/mod.rs and use it in identity_map()

src/memory/mod.rs

@@ -1,4 +1,5 @@
 pub use self::area_frame_allocator::AreaFrameAllocator;
+use self::paging::PhysicalAddress;
 
 pub mod paging;
 mod area_frame_allocator;
@@ -14,6 +15,10 @@ impl Frame {
     fn containing_address(address: usize) -> Frame {
         Frame { number: address / PAGE_SIZE }
     }
+
+    fn start_address(&self) -> PhysicalAddress {
+        self.number * PAGE_SIZE
+    }
 }
 
 pub trait FrameAllocator {