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`.
* Introduce a `Slot` type
Instead of having some type definition that only was used in half of the
code or directly using `u64`, this adds a new unit type wrapper `Slot`.
This makes it especially easy for the outside api to know what type is
expected/returned.
* Change epoch duratioC
* rename all instances of slot number to slot
* Make the constructor private
Co-authored-by: André Silva <andrerfosilva@gmail.com>
Bastian Köcher