Aura and Slots refactoring (#8386)

* Make slot duration being exposed as `Duration` to the outside

* Some slot info love

* Add `build_aura_worker` utility function

* Copy copy copy

substrate/bin/node-template/runtime/src/lib.rs

@@ -378,8 +378,8 @@ impl_runtime_apis! {
 	}
 
 	impl sp_consensus_aura::AuraApi<Block, AuraId> for Runtime {
-		fn slot_duration() -> u64 {
-			Aura::slot_duration()
+		fn slot_duration() -> sp_consensus_aura::SlotDuration {
+			sp_consensus_aura::SlotDuration::from_millis(Aura::slot_duration())
 		}
 
 		fn authorities() -> Vec<AuraId> {