Create a memory::paging::entry submodule with an Entry struct

src/memory/paging/entry.rs

@@ -0,0 +1,13 @@
+use memory::Frame;
+
+pub struct Entry(u64);
+
+impl Entry {
+    pub fn is_unused(&self) -> bool {
+        self.0 == 0
+    }
+
+    pub fn set_unused(&mut self) {
+        self.0 = 0;
+    }
+}

src/memory/paging/mod.rs

@@ -1,5 +1,7 @@
 use memory::PAGE_SIZE;
 
+mod entry;
+
 const ENTRY_COUNT: usize = 512;
 
 pub type PhysicalAddress = usize;