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/client/consensus/pow/src/lib.rs

@@ -352,7 +352,7 @@ impl<B, I, C, S, Algorithm, CAW> BlockImport<B> for PowBlockImport<B, I, C, S, A
 				check_block.clone(),
 				BlockId::Hash(parent_hash),
 				inherent_data,
-				timestamp_now
+				*timestamp_now,
 			)?;
 
 			block.body = Some(check_block.deconstruct().1);