Breaking Change: Improved `VirtAddr` & `PhysAddr`

[jounathaen]

Make VirtAddr and PhysAddr behave more similar to x86_64:

• Added a new fn with same signature
• Address Arithmetic modifications:
  • Addition of two addresses makes no sense
  • Difference of two addresses should yield a u64, not an address
  • Opinionated: Arithmetics with usize shall not exist for addresses to avoid inconsistent behaviour on 32/64-bit systems. Be explicit when using usizes