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/primitives/consensus/babe/src/digests.rs

@@ -19,7 +19,7 @@
 
 use super::{
 	AllowedSlots, AuthorityId, AuthorityIndex, AuthoritySignature, BabeAuthorityWeight,
-	BabeEpochConfiguration, SlotNumber, BABE_ENGINE_ID,
+	BabeEpochConfiguration, Slot, BABE_ENGINE_ID,
 };
 use codec::{Codec, Decode, Encode};
 use sp_std::vec::Vec;
@@ -32,8 +32,8 @@ use sp_consensus_vrf::schnorrkel::{Randomness, VRFOutput, VRFProof};
 pub struct PrimaryPreDigest {
 	/// Authority index
 	pub authority_index: super::AuthorityIndex,
-	/// Slot number
-	pub slot_number: SlotNumber,
+	/// Slot
+	pub slot: Slot,
 	/// VRF output
 	pub vrf_output: VRFOutput,
 	/// VRF proof
@@ -50,8 +50,8 @@ pub struct SecondaryPlainPreDigest {
 	/// it makes things easier for higher-level users of the chain data to
 	/// be aware of the author of a secondary-slot block.
 	pub authority_index: super::AuthorityIndex,
-	/// Slot number
-	pub slot_number: SlotNumber,
+	/// Slot
+	pub slot: Slot,
 }
 
 /// BABE secondary deterministic slot assignment with VRF outputs.
@@ -59,8 +59,8 @@ pub struct SecondaryPlainPreDigest {
 pub struct SecondaryVRFPreDigest {
 	/// Authority index
 	pub authority_index: super::AuthorityIndex,
-	/// Slot number
-	pub slot_number: SlotNumber,
+	/// Slot
+	pub slot: Slot,
 	/// VRF output
 	pub vrf_output: VRFOutput,
 	/// VRF proof
@@ -93,12 +93,12 @@ impl PreDigest {
 		}
 	}
 
-	/// Returns the slot number of the pre digest.
-	pub fn slot_number(&self) -> SlotNumber {
+	/// Returns the slot of the pre digest.
+	pub fn slot(&self) -> Slot {
 		match self {
-			PreDigest::Primary(primary) => primary.slot_number,
-			PreDigest::SecondaryPlain(secondary) => secondary.slot_number,
-			PreDigest::SecondaryVRF(secondary) => secondary.slot_number,
+			PreDigest::Primary(primary) => primary.slot,
+			PreDigest::SecondaryPlain(secondary) => secondary.slot,
+			PreDigest::SecondaryVRF(secondary) => secondary.slot,
 		}
 	}