Refactor DigestItem (#2108)

* Add `start_aura2`.

* .gitignore patch conflict files

and remove one that accidentally got committed

* Fix build

The tests still don’t work.

* Fix compilation errors

* Fix compile errors (again)

* Try (and fail) to fix tests

* Properly deserialize data

Previously, `DigestItem::Consensus` had no separate `DigestItemType`,
so it did not get properly serialized and deserialized.

* Add extra debug logging.  Always allow old seals.

A `RUST_LOG=substrate_aura_consensus cargo test --all -- --nocapture \
tests::authoring_blocks` revealed that old seals were being and
rejected, causing the test to hang.  As a temporary debug measure, allow
old seals unconditionally, so that CI can test if this fixes the
problem.

* Forcibly disable rejection of old seals

* Use old trait, but newer serialization

The old trait for `CompatibleDigestItem` actually worked.  By changing
its implementation, one can ensure that all *new* seals have the modern
form, but *legacy* seals are still decoded correctly.

* Bump impl version

* Squash spurious deprecation warning

`rustc` should not be emitting a deprecation warning in deprecated
code, but it does, so silence it.

* Rip out unused Cargo feature

* Move AURA to aura_primitives

* Respond to code review

* Wrap overly-long line

* Reduce logging verbosity and add target

* Add dependency on `sr-primitives` to `aura_primitives`

* Fix build

It failed with a message about Cargo.lock being out of date.

* core: aura: rename aura engine id const

* core: aura: remove superfluous logging

* core: primitives: add removed semicolons

* core: aura: remove unused import

* core: network: style fix

* runtime: update wasm blobs

* runtime: bump impl_version

* core: primitives: tag all DigestItemType variants explicitly

substrate/core/service/src/lib.rs

@@ -48,6 +48,7 @@ pub use chain_spec::{ChainSpec, Properties};
 pub use transaction_pool::txpool::{
 	self, Pool as TransactionPool, Options as TransactionPoolOptions, ChainApi, IntoPoolError
 };
+use client::runtime_api::BlockT;
 pub use client::FinalityNotifications;
 
 pub use components::{ServiceFactory, FullBackend, FullExecutor, LightBackend,
@@ -227,9 +228,9 @@ impl<Components: components::Components> Service<Components> {
 			// A utility stream that drops all ready items and only returns the last one.
 			// This is used to only keep the last finality notification and avoid
 			// overloading the sync module with notifications.
-			struct MostRecentNotification<B: network::BlockT>(futures::stream::Fuse<FinalityNotifications<B>>);
+			struct MostRecentNotification<B: BlockT>(futures::stream::Fuse<FinalityNotifications<B>>);
 
-			impl<B: network::BlockT> Stream for MostRecentNotification<B> {
+			impl<B: BlockT> Stream for MostRecentNotification<B> {
 				type Item = <FinalityNotifications<B> as Stream>::Item;
 				type Error = <FinalityNotifications<B> as Stream>::Error;