Convert timestamp to unit type wrapper (#8333)

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`.

substrate/frame/babe/src/lib.rs

@@ -25,7 +25,7 @@ use codec::{Decode, Encode};
 use frame_support::{
 	decl_error, decl_module, decl_storage,
 	dispatch::DispatchResultWithPostInfo,
-	traits::{FindAuthor, Get, KeyOwnerProofSystem, OneSessionHandler},
+	traits::{FindAuthor, Get, KeyOwnerProofSystem, OneSessionHandler, OnTimestampSet},
 	weights::{Pays, Weight},
 	Parameter,
 };
@@ -38,7 +38,6 @@ use sp_runtime::{
 };
 use sp_session::{GetSessionNumber, GetValidatorCount};
 use sp_std::prelude::*;
-use sp_timestamp::OnTimestampSet;
 
 use sp_consensus_babe::{
 	digests::{NextConfigDescriptor, NextEpochDescriptor, PreDigest},