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/srml/balances/src/lib.rs

@@ -423,6 +423,23 @@ impl<T: Trait> Module<T> {
 		Self::set_free_balance_creating(who, Self::free_balance(who) + value)
 	}
 
+	/// Substrates `value` from the free balance of `who`. If the whole amount cannot be
+	/// deducted, an error is returned.
+	///
+	/// NOTE: This assumes that the total stake remains unchanged after this operation. If
+	/// you mean to actually burn value out of existence, then use `slash` instead.
+	pub fn decrease_free_balance(
+		who: &T::AccountId,
+		value: T::Balance
+	) -> result::Result<UpdateBalanceOutcome, &'static str> {
+		T::EnsureAccountLiquid::ensure_account_liquid(who)?;
+		let b = Self::free_balance(who);
+		if b < value {
+			return Err("account has too few funds")
+		}
+		Ok(Self::set_free_balance(who, b - value))
+	}
+
 	/// Deducts up to `value` from the combined balance of `who`, preferring to deduct from the
 	/// free balance. This function cannot fail.
 	///

substrate/srml/balances/src/tests.rs

@@ -280,6 +280,17 @@ fn balance_transfer_works() {
 	});
 }
 
+#[test]
+fn balance_reduction_works() {
+	with_externalities(&mut ExtBuilder::default().build(), || {
+		Balances::set_free_balance(&1, 111);
+		Balances::increase_total_stake_by(111);
+		assert_ok!(Balances::decrease_free_balance(&1, 69).map(|_| ()));
+		assert_eq!(Balances::total_balance(&1), 42);
+		assert_noop!(Balances::decrease_free_balance(&1, 69).map(|_| ()), "account has too few funds");
+	});
+}
+
 #[test]
 fn reserving_balance_should_work() {
 	with_externalities(&mut ExtBuilder::default().build(), || {