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/core/sr-primitives/src/generic/digest.rs

@@ -74,13 +74,14 @@ pub enum DigestItem<Hash, AuthorityId, SealSignature> {
 	/// be generated by the native code of any consensus engine, but this is not
 	/// checked (yet).
 	Consensus(ConsensusEngineId, Vec<u8>),
-	/// Put a Seal on it.
+	/// Put a Seal on it. This is only used by native code, and is never seen
+	/// by runtimes.
 	Seal(ConsensusEngineId, SealSignature),
 	/// A pre-runtime digest.
 	///
 	/// These are messages from the consensus engine to the runtime, although
 	/// the consensus engine can (and should) read them itself to avoid
-	/// code and state duplication.  It is erroneous for a runtime to produce
+	/// code and state duplication. It is erroneous for a runtime to produce
 	/// these, but this is not (yet) checked.
 	PreRuntime(ConsensusEngineId, Vec<u8>),
 	/// Any 'non-system' digest item, opaque to the native code.
@@ -110,7 +111,8 @@ pub enum DigestItemRef<'a, Hash: 'a, AuthorityId: 'a, SealSignature: 'a> {
 	/// be generated by the native code of any consensus engine, but this is not
 	/// checked (yet).
 	Consensus(&'a ConsensusEngineId, &'a Vec<u8>),
-	/// Put a Seal on it.
+	/// Put a Seal on it. This is only used by native code, and is never seen
+	/// by runtimes.
 	Seal(&'a ConsensusEngineId, &'a SealSignature),
 	/// A pre-runtime digest.
 	///