Introduce a Slot type (#7997)

* 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>

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

@@ -1200,7 +1200,7 @@ impl_runtime_apis! {
 			}
 		}
 
-		fn current_epoch_start() -> sp_consensus_babe::SlotNumber {
+		fn current_epoch_start() -> sp_consensus_babe::Slot {
 			Babe::current_epoch_start()
 		}
 
@@ -1213,7 +1213,7 @@ impl_runtime_apis! {
 		}
 
 		fn generate_key_ownership_proof(
-			_slot_number: sp_consensus_babe::SlotNumber,
+			_slot: sp_consensus_babe::Slot,
 			authority_id: sp_consensus_babe::AuthorityId,
 		) -> Option<sp_consensus_babe::OpaqueKeyOwnershipProof> {
 			use codec::Encode;