BABE Randomness using PreRuntime digests (#2929)

* Initial work on exposing pre-runtime digests

This provides the primitive API, as well as exposing it from BABE.

* Initial work on using pre-digests in runtimes

This includes both code to expose them from `srml_system`, as well as
using it in (currently dead) code in `srml_babe`.

* Bump `{spec,impl}_version`

* Add `u64_backend` feature to curve25519-dalek

Otherwise, it errors out at compile-time.

* Bump `Cargo.lock`

* Do not depend on the schnorrkel crate in the runtime

The schnorrkel crate does not work on `#![no_std]`, but the runtime only
needs constants from it.  This adds our own definitions of those
constants, and checks them for correctness at compile-time.

* Actually implement storage of VRF outputs

* Trivial formatting change

* Provide a `hash_randomness` function in BABE

for processing VRF outputs.

* Implement a basic randomness generating function

It just XORs the VRF outputs together.

* Actually implement on-chain randomness

Blake2b is used for hashing.

* Update dependencies

* Run `cargo update` where needed

* Re-add a newline at EOF

* Remove broken and unsafe code

XOR is not a hash function, and must not be used as such.  The
implementation was also needlessly unsafe.

* Run `cargo update` where needed

* Remove spurious dependency

* Document security guarantees of BABE randomness

* Add a `RandomnessBeacon` trait

* Document `RandomnessBeacon::random`

* Fix silly compile error (unexpected type arguments)

* Fix BABE randomness

* Implement `FindAuthor` for `babe::Module`

* Apply suggestions from code review

Co-Authored-By: Bastian Köcher <bkchr@users.noreply.github.com>
Co-Authored-By: Robert Habermeier <rphmeier@gmail.com>

* Respond to suggestions from code review and fix bugs

* Store an authority index, not the authority itself.
* Avoid unnecessary decoding.
* Implement relative slots and BABE randomness fully and correctly.

* Remove spurious dependency

* Fix error reported by rust-analyzer

* Update Cargo.lock files

* `wrapping_add` → `checked_add`

The epoch index will not overflow.  Panic if it does.

* Move randomness documentation to trait

* Fix compile error in test suite

* Explain 2^64 limit

Co-Authored-By: Robert Habermeier <rphmeier@gmail.com>

substrate/core/consensus/slots/src/lib.rs

@@ -48,10 +48,6 @@ pub trait SlotWorker<B: Block> {
 	/// triggered.
 	type OnSlot: IntoFuture<Item = (), Error = consensus_common::Error>;
 
-	/// Called when the proposer starts.
-	#[deprecated(note = "Not called. Please perform any initialization before calling start_slot_worker.")]
-	fn on_start(&self, _slot_duration: u64) -> Result<(), consensus_common::Error> { Ok(()) }
-
 	/// Called when a new slot is triggered.
 	fn on_slot(&self, chain_head: B::Header, slot_info: SlotInfo) -> Self::OnSlot;
 }
@@ -60,8 +56,13 @@ pub trait SlotWorker<B: Block> {
 pub trait SlotCompatible {
 	/// Extract timestamp and slot from inherent data.
 	fn extract_timestamp_and_slot(
+		&self,
 		inherent: &InherentData,
-	) -> Result<(u64, u64), consensus_common::Error>;
+	) -> Result<(u64, u64, std::time::Duration), consensus_common::Error>;
+
+	/// Get the difference between chain time and local time.  Defaults to
+	/// always returning zero.
+	fn time_offset() -> SignedDuration { Default::default() }
 }
 
 /// Start a new slot worker.
@@ -74,6 +75,7 @@ pub fn start_slot_worker<B, C, W, T, SO, SC>(
 	worker: W,
 	sync_oracle: SO,
 	inherent_data_providers: InherentDataProviders,
+	timestamp_extractor: SC,
 ) -> impl Future<Item = (), Error = ()>
 where
 	B: Block,
@@ -86,8 +88,11 @@ where
 	let SlotDuration(slot_duration) = slot_duration;
 
 	// rather than use a timer interval, we schedule our waits ourselves
-	let mut authorship = Slots::<SC>::new(slot_duration.slot_duration(), inherent_data_providers)
-		.map_err(|e| debug!(target: "slots", "Faulty timer: {:?}", e))
+	let mut authorship = Slots::<SC>::new(
+		slot_duration.slot_duration(),
+		inherent_data_providers,
+		timestamp_extractor,
+	).map_err(|e| debug!(target: "slots", "Faulty timer: {:?}", e))
 		.for_each(move |slot_info| {
 			// only propose when we are not syncing.
 			if sync_oracle.is_major_syncing() {