Small differences between old-base and new-base.

[Philip-NLnetLabs]

This issue tracks small differences between old-base and new-base where the difference needlessly adds work to migrating from old-base to new-base. Just add items here and mark them as resolved when new-base has been updated in the main branch.

• Serial method add became inc

  domain::base::serial::Serial has a method add(self, u32) while domain::new::base::serial::Serial has a method inc(self, i32).
  i32 might be better that u32 but renaming from add to inc is unneeded.

• Serial method now became unix_time

  domain::base::serial::Serial vs domain::new::base::serial::Serial.
  The fn doc string was changed as well.

• Rtype became RType

  domain::base::iana::rtype::Rtype vs domain::base::iana::RType