Consensus Engines Implementation: Aura (#911)

* Generalize BlockImport

 - move ImportBlock, BlockOrigin, ImportResult into shared sr-primitives
 - let Consensus provide  and  traits again
 - update consensus traits to latest development
 - implement traits on client::Client, test_client::TestClient
 - update RHD to use the new import_block API

* Move ImportBlock into consensus-common
* Send import notification in aura tests
* Integrating aura into service
* Make Signatures more generic
* Aura Block Production with the given key
* run aura on the thread pool
* start at exact step start in aura
* Add needed wasm blob, in leiu of better solutions.
* Make API ids consistent with traits and bring upstream for sharing.
* Add decrease_free_balance to Balances module
* Encode `Metadata` once instead of two times
* Bitops include xor
* Upgrade key module.
* Default pages to somewhat bigger.
* Introduce upgrade key into node
* Add `Created` event

substrate/core/sr-primitives/src/testing.rs

@@ -42,6 +42,10 @@ impl traits::Digest for Digest {
 	fn push(&mut self, item: Self::Item) {
 		self.logs.push(item);
 	}
+
+	fn pop(&mut self) -> Option<Self::Item> {
+		self.logs.pop()
+	}
 }
 
 #[derive(PartialEq, Eq, Clone, Serialize, Deserialize, Debug, Encode, Decode)]
@@ -74,6 +78,7 @@ impl traits::Header for Header {
 	fn set_parent_hash(&mut self, hash: Self::Hash) { self.parent_hash = hash }
 
 	fn digest(&self) -> &Self::Digest { &self.digest }
+	fn digest_mut(&mut self) -> &mut Self::Digest { &mut self.digest }
 	fn set_digest(&mut self, digest: Self::Digest) { self.digest = digest }
 
 	fn new(