Use an VirtAddress/PhysAddress type instead of usize

[jounathaen]

As the types in this crate can be used for managing (allocating and deallocating) physical and virtual memory, it might be reasonable to use specific types for this, especially for PageRange. I have made an attempt to provide such types in https://github.com/hermit-os/memory-addresses, maybe they are useful here or can be improved to be used in this crate.