Strongly typed virtual/physical memory seperation.

This minicommit introduces two newtpyes, `Physical` and `Virtual`,
respectively. These serves as a way to segregate the different forms of
addresses to avoid the issues we had in the old kernel.

kernel/arch/x86_64/mod.rs

@@ -23,3 +23,5 @@ pub mod serial;
 
 /// Task state segment.
 pub mod tss;
+
+pub mod physical;

kernel/arch/x86_64/physical.rs

@@ -0,0 +1,11 @@
+//! Typestrong address segregation.
+
+/// A physical address in memory.
+#[derive(Clone, Debug, PartialEq, Eq)]
+pub struct Physical {
+    /// The position.
+    ///
+    /// Note that we do not use a pointer here to avoid simple mistakes where the programmer
+    /// confuse virtual and physical.
+    pub inner: u64,
+}