Add runtime support for PreRuntime and Consensus digests (#2757)

* Try to fix runtime panic

Does not work

* Fix trivial typo

* Add runtime support for `PreRuntime` and `Consensus` digests

Fixes c7d1204ce5

* Fix silly compile error.

* Fix overly-long lines

Also remove some in-progress code that would not wind up being useful
anyway.

* Respond to review comments

* delete `unset RUSTC_WRAPPER` from scripts/common.sh
* delete unnecessary `use aura::AURA_ENGINE_ID` from
  `node/runtime/src/lib.rs`
* add comments explaining why `PreRuntime` and `Consensus` must be
  special-cased in `core/sr-primitives/lib.rs`
* switch to using `$crate::rstd::marker::PhantomData` in
  `impl_outer_log!`
* improve documentation of `DigestItem::Seal`

* Fix compilation and add proof that we do not panic

Also fix some warnings.

* Apply suggestions from code review

Mostly for readability

Co-Authored-By: Sergei Pepyakin <s.pepyakin@gmail.com>

* Apply suggestions from code review

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

* $crate::rstd::marker::PhantomData → Default::default()

The import is still needed, as `Default::default()` can’t be used in
patterns.

* Bump `spec_version`

Also do some reformatting.

substrate/srml/aura/src/lib.rs

@@ -51,15 +51,19 @@
 pub use timestamp;
 
 use rstd::{result, prelude::*};
+use parity_codec::{Encode, Decode};
 use srml_support::storage::StorageValue;
 use srml_support::{decl_storage, decl_module};
 use primitives::traits::{SaturatedConversion, Saturating, Zero, One};
 use timestamp::OnTimestampSet;
+use rstd::marker::PhantomData;
 #[cfg(feature = "std")]
 use timestamp::TimestampInherentData;
 use inherents::{RuntimeString, InherentIdentifier, InherentData, ProvideInherent, MakeFatalError};
 #[cfg(feature = "std")]
 use inherents::{InherentDataProviders, ProvideInherentData};
+#[cfg(feature = "std")]
+use serde::Serialize;
 
 mod mock;
 mod tests;
@@ -89,6 +93,20 @@ impl AuraInherentData for InherentData {
 	}
 }
 
+/// Logs in this module.
+pub type Log<T> = RawLog<T>;
+
+/// Logs in this module.
+///
+/// The type parameter distinguishes logs belonging to two different runtimes,
+/// which should not be mixed.
+#[cfg_attr(feature = "std", derive(Serialize, Debug))]
+#[derive(Encode, Decode, PartialEq, Eq, Clone)]
+pub enum RawLog<T> {
+	/// AuRa inherent digests
+	PreRuntime([u8; 4], Vec<u8>, PhantomData<T>),
+}
+
 /// Provides the slot duration inherent data for `Aura`.
 #[cfg(feature = "std")]
 pub struct InherentDataProvider {