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;