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 <[email protected]>
Co-Authored-By: Robert Habermeier <[email protected]>

* 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 <[email protected]>