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/support/src/traits.rs

@@ -2291,6 +2291,13 @@ pub trait ExecuteBlock<Block: BlockT> {
 	fn execute_block(block: Block);
 }
 
+/// A trait which is called when the timestamp is set in the runtime.
+#[impl_trait_for_tuples::impl_for_tuples(30)]
+pub trait OnTimestampSet<Moment> {
+	/// Called when the timestamp is set.
+	fn on_timestamp_set(moment: Moment);
+}
+
 #[cfg(test)]
 mod tests {
 	use super::*;