Introduce inherent digests (#2466)

* Introduce inherent digests

* Implement inherent digests

* fix silly error

* Implementation of inherent digests in BABE

All tests pass. There are still limitations:

1. The runtime strips out inherent digests, so BABE must re-add them.
2. The test runtime checks that it can re-compute all digests.  It
   can’t, so I had to comment out that test.

* Fix compilation and seal import

Seals were not imported correctly: the pre-digest was imported twice,
instead of both it and the seal being imported.  Also, other parts of
the code did not compile due to incomplete refactoring.

* Remove bogus assertion

* Fix testsuite compilation

* Remove unused import

* Fix compiler diagnostics

* Add inherent digest parameters to block constructors

This enforces that inherent digests are added first.

* Fixup Cargo.lock

* Fix build errors

* Re-add an incorrectly removed import

* Bump primitive-types version

* Update Cargo.lock

* Refactoring

* Use inherent digests for AuRa

They do reach the runtime, but get stripped.  I have not figured out
where.

* Fix compilation errors

* Fix compilation errors due to incorrect types

* Fix whitespace

Suggested-by: Tomasz Drwiega <tomasz@parity.io>

* Add preamble

Suggested-by: Tomasz Drwiega <tomasz@parity.io>

* Fix silly compile error

* Refactor pre-digest finding code into a separate function

* Remove unwanted assertion

It is too likely to bring down the entire blockchain.

Suggested-by: Tomasz Drwiega <tomasz@parity.io>

* Use `find_pre_digest` after runtime, too

Also, use `Member` trait rather than rolling our own requirements.

Suggested-by: Tomasz Drwiega <tomasz@parity.io>

* Fix various warnings

mostly due to upgrading the dependency on `error_chain`.

* Pre-digests nearly complete

This nearly completes the implementation of pre-runtime digests.

* `Seal2` → `Seal` and fix test suite

* Try to fix the storage error

* Try to fix storage (again)

* Fix tests

* Hopefully finish pre-runtime digests

The key is to pass *only* the pre-runtime digests to the runtime.  The
others must be stripped out by `initialize_block`.

* Fix silly typo

* Fix another silly mistake

* Remove unnecessary filtering of BABE pre-digests

We no longer get duplicate BABE pre-digests, so if they appear, the
header should be rejected outright.

* Update Cargo.lock files

* Reformatting

* Fix silly typo in inherent digest code

Also, revert `error.rs` files that contained calls to the `error_chain!`
macro.

* Try to keep the runtime from stripping pre-digests

Currently runs into the “Storage root must match that calculated”
assertion.

* Don’t compute storage root until storage changes are done.

Also, fix a compilation error.

* Fix compile-time error

* Fix compilation errors

* Fix more compile errors

* Hopefully it compiles this time…

* Fix compilation and add docs

* Prevent BABE from adding duplicate pre-runtime digests

Found by comparing with the AuRa code.  I also did some refactoring.

* Respond to review and fix some warnings

* Delete some dead code introduced earlier

* More dead code goes away

* `ref mut` → `&mut`

* Respond to review and fix some warnings

* Fix compilation error

* Remove unneeded `HashT` type parameter

Suggested-by: Robert Habermeier <robert@parity.io>

* Remove spurious #[allow(deprecated)]

* Document inherent digest parameter to `build_block`

* Delete `Simple` trait

It wasn’t needed

* delete wrongly added files

* Fix trait bounds

* Digest serialization tests

I also did some reformatting and cleanup.

* Apply suggestions from code review

Reformatting

Co-Authored-By: André Silva <andre.beat@gmail.com>

* Swap two arguments to `propose` and `propose_with`

Also, remove some needless unsafe code.

* Remove bogus `#![allow(deprecated)]` annotations

With the removal of the deprecated `Seal` variant, these are not needed.

* Add a missing `#[allow(deprecated)]` in the AuRa tests

* Fix silly compile error

* Fix silly compiler error

RLS did not tell me that I hadn’t fixed `babe/lib.rs`, so I missed it.

* Fixes made automatically by Cargo

substrate/core/client/src/block_builder/block_builder.rs

@@ -20,7 +20,7 @@ use parity_codec::Encode;
 use runtime_primitives::ApplyOutcome;
 use runtime_primitives::generic::BlockId;
 use runtime_primitives::traits::{
-	Header as HeaderT, Hash, Block as BlockT, One, HashFor, ProvideRuntimeApi, ApiRef
+	Header as HeaderT, Hash, Block as BlockT, One, HashFor, ProvideRuntimeApi, ApiRef, DigestFor,
 };
 use primitives::{H256, ExecutionContext};
 use crate::blockchain::HeaderBackend;
@@ -41,10 +41,11 @@ where
 	A: ProvideRuntimeApi + HeaderBackend<Block> + 'a,
 	A::Api: BlockBuilderApi<Block>,
 {
-	/// Create a new instance of builder from the given client, building on the latest block.
-	pub fn new(api: &'a A) -> error::Result<Self> {
+	/// Create a new instance of builder from the given client, building on the
+	/// latest block.
+	pub fn new(api: &'a A, inherent_digests: DigestFor<Block>) -> error::Result<Self> {
 		api.info().and_then(|i|
-			Self::at_block(&BlockId::Hash(i.best_hash), api, false)
+			Self::at_block(&BlockId::Hash(i.best_hash), api, false, inherent_digests)
 		)
 	}
 
@@ -57,7 +58,8 @@ where
 	pub fn at_block(
 		block_id: &BlockId<Block>,
 		api: &'a A,
-		proof_recording: bool
+		proof_recording: bool,
+		inherent_digests: DigestFor<Block>,
 	) -> error::Result<Self> {
 		let number = api.block_number_from_id(block_id)?
 			.ok_or_else(|| error::Error::UnknownBlock(format!("{}", block_id)))?
@@ -70,7 +72,7 @@ where
 			Default::default(),
 			Default::default(),
 			parent_hash,
-			Default::default()
+			inherent_digests,
 		);
 
 		let mut api = api.runtime_api();
@@ -80,7 +82,7 @@ where
 		}
 
 		api.initialize_block_with_context(
-			block_id, ExecutionContext::BlockConstruction, &header
+			block_id, ExecutionContext::BlockConstruction, &header,
 		)?;
 
 		Ok(BlockBuilder {