The timestamp inherent type was up to now just a simple `u64`. This
worked, but doesn't give you that much guarantees at compile time about
the type. This pr changes that by converting this type to a unit type
wrapper, similar to what we have done for `Slot`.

This is required for some future pr that touches quite a lot of the
inherents stuff :)

Besides this unit wrapper type, this pr also moves the `OnTimestampSet`
trait to `frame_support::traits`.