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/aura/src/lib.rs

@@ -117,12 +117,14 @@ struct AuraSlotCompatible;
 
 impl SlotCompatible for AuraSlotCompatible {
 	fn extract_timestamp_and_slot(
+		&self,
 		data: &InherentData
-	) -> Result<(TimestampInherent, AuraInherent), consensus_common::Error> {
+	) -> Result<(TimestampInherent, AuraInherent, std::time::Duration), consensus_common::Error> {
 		data.timestamp_inherent_data()
 			.and_then(|t| data.aura_inherent_data().map(|a| (t, a)))
 			.map_err(Into::into)
 			.map_err(consensus_common::Error::InherentData)
+			.map(|(x, y)| (x, y, Default::default()))
 	}
 }
 
@@ -170,7 +172,8 @@ pub fn start_aura<B, C, SC, E, I, P, SO, Error, H>(
 		select_chain,
 		worker,
 		sync_oracle,
-		inherent_data_providers
+		inherent_data_providers,
+		AuraSlotCompatible,
 	))
 }
 
@@ -514,7 +517,7 @@ impl<B: Block, C, P> Verifier<B> for AuraVerifier<C, P> where
 		mut body: Option<Vec<B::Extrinsic>>,
 	) -> Result<(ImportBlock<B>, Option<Vec<(CacheKeyId, Vec<u8>)>>), String> {
 		let mut inherent_data = self.inherent_data_providers.create_inherent_data().map_err(String::from)?;
-		let (timestamp_now, slot_now) = AuraSlotCompatible::extract_timestamp_and_slot(&inherent_data)
+		let (timestamp_now, slot_now, _) = AuraSlotCompatible.extract_timestamp_and_slot(&inherent_data)
 			.map_err(|e| format!("Could not extract timestamp and slot: {:?}", e))?;
 		let hash = header.hash();
 		let parent_hash = *header.parent_hash();