This defines Pxe / Pfn types that makes it easier to manipulate PFNs
and PXEs.
§Examples
let pxe = Pxe::new(
Pfn::new(0x6d600),
PxeFlags::USER_ACCESSIBLE | PxeFlags::ACCESSED | PxeFlags::PRESENT
);
let encoded = u64::from(pxe);
let decoded = Pxe::from(encoded);