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/Cargo.lock

@@ -6724,6 +6724,7 @@ dependencies = [
  "sp-blockchain",
  "sp-consensus",
  "sp-consensus-aura",
+ "sp-consensus-slots",
  "sp-core",
  "sp-inherents",
  "sp-io",
@@ -6776,6 +6777,7 @@ dependencies = [
  "sp-blockchain",
  "sp-consensus",
  "sp-consensus-babe",
+ "sp-consensus-slots",
  "sp-consensus-vrf",
  "sp-core",
  "sp-inherents",
@@ -6856,6 +6858,7 @@ dependencies = [
  "sp-blockchain",
  "sp-consensus",
  "sp-consensus-babe",
+ "sp-consensus-slots",
  "sp-core",
  "sp-inherents",
  "sp-keyring",
@@ -8247,6 +8250,7 @@ dependencies = [
  "parity-scale-codec",
  "sp-api",
  "sp-application-crypto",
+ "sp-consensus-slots",
  "sp-inherents",
  "sp-runtime",
  "sp-std",
@@ -8288,6 +8292,7 @@ name = "sp-consensus-slots"
 version = "0.8.1"
 dependencies = [
  "parity-scale-codec",
+ "sp-arithmetic",
  "sp-runtime",
 ]