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/babe/Cargo.toml

@@ -29,7 +29,6 @@ sp-runtime = { version = "3.0.0", default-features = false, path = "../../primit
 sp-session = { version = "3.0.0", default-features = false, path = "../../primitives/session" }
 sp-staking = { version = "3.0.0", default-features = false, path = "../../primitives/staking" }
 sp-std = { version = "3.0.0", default-features = false, path = "../../primitives/std" }
-sp-timestamp = { version = "3.0.0", default-features = false, path = "../../primitives/timestamp" }
 log = { version = "0.4.14", default-features = false }
 
 [dev-dependencies]
@@ -59,7 +58,6 @@ std = [
 	"sp-session/std",
 	"sp-staking/std",
 	"sp-std/std",
-	"sp-timestamp/std",
 	"log/std",
 ]
 runtime-benchmarks = ["frame-benchmarking"]

substrate/frame/babe/src/lib.rs

@@ -25,7 +25,7 @@ use codec::{Decode, Encode};
 use frame_support::{
 	decl_error, decl_module, decl_storage,
 	dispatch::DispatchResultWithPostInfo,
-	traits::{FindAuthor, Get, KeyOwnerProofSystem, OneSessionHandler},
+	traits::{FindAuthor, Get, KeyOwnerProofSystem, OneSessionHandler, OnTimestampSet},
 	weights::{Pays, Weight},
 	Parameter,
 };
@@ -38,7 +38,6 @@ use sp_runtime::{
 };
 use sp_session::{GetSessionNumber, GetValidatorCount};
 use sp_std::prelude::*;
-use sp_timestamp::OnTimestampSet;
 
 use sp_consensus_babe::{
 	digests::{NextConfigDescriptor, NextEpochDescriptor, PreDigest},