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/paging/mod.rs

@@ -0,0 +1,6 @@
+/// A newtype representing a virtual address.
+#[derive(Copy, Clone, Debug, Eq, Ord, PartialEq, PartialOrd)]
+pub struct Virtual {
+    /// The inner value.
+    pub inner: usize,
+}