Use newtype for physical and virtual addresses

[Timmmm]

For clarify and to avoid mixing them up, we can use Sail's newtype feature to create distinct types for physical and virtual addresses. For example:

newtype physaddr = physaddr : xlenbits
newtype virtaddr = virtaddr : xlenbits

function translate(virtaddr(vaddr) : virtaddr) -> physaddr = physaddr(vaddr + 1)

function main() -> unit = {
  let x = physaddr(zeros(32));

  let y = translate(x)
}

This will give a compile time error since we accidentally passed a physical address into translate().

See #433