Add StorageValue::append and speed-up deposit_event (#2282)

* Adds deposit event benchmark

* Add `StorageValue::append`

`StorageValue::append` can be used by types that implement `EncodeAppend` to speed-up situations where you just want to append
an item to storage without wanting to decode all previous items.

* Stay at 100 events

* Fixes compilation

* Use correct year and increase spec version

substrate/srml/system/benches/bench.rs

@@ -0,0 +1,99 @@
+// Copyright 2019 Parity Technologies (UK) Ltd.
+// This file is part of Substrate.
+
+// Substrate is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Substrate is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with Substrate.  If not, see <http://www.gnu.org/licenses/>.
+
+use criterion::{Criterion, criterion_group, criterion_main, black_box};
+use srml_system as system;
+use srml_support::{decl_module, decl_event, impl_outer_origin, impl_outer_event};
+use runtime_io::{with_externalities, Blake2Hasher};
+use substrate_primitives::H256;
+use primitives::{
+	BuildStorage, traits::{BlakeTwo256, IdentityLookup},
+	testing::{Digest, DigestItem, Header},
+};
+
+mod module {
+	use super::*;
+
+	pub trait Trait: system::Trait {
+		type Event: From<Event> + Into<<Self as system::Trait>::Event>;
+	}
+
+	decl_module! {
+		pub struct Module<T: Trait> for enum Call where origin: T::Origin {
+			pub fn deposit_event() = default;
+		}
+	}
+
+	decl_event!(
+		pub enum Event {
+			Complex(Vec<u8>, u32, u16, u128),
+		}
+	);
+}
+
+impl_outer_origin!{
+	pub enum Origin for Runtime {}
+}
+
+impl_outer_event! {
+	pub enum Event for Runtime {
+		module,
+	}
+}
+
+#[derive(Clone, Eq, PartialEq)]
+pub struct Runtime;
+impl system::Trait for Runtime {
+	type Origin = Origin;
+	type Index = u64;
+	type BlockNumber = u64;
+	type Hash = H256;
+	type Hashing = BlakeTwo256;
+	type Digest = Digest;
+	type AccountId = u64;
+	type Lookup = IdentityLookup<Self::AccountId>;
+	type Header = Header;
+	type Event = Event;
+	type Log = DigestItem;
+}
+
+impl module::Trait for Runtime {
+	type Event = Event;
+}
+
+fn new_test_ext() -> runtime_io::TestExternalities<Blake2Hasher> {
+	system::GenesisConfig::<Runtime>::default().build_storage().unwrap().0.into()
+}
+
+fn deposit_events(n: usize) {
+	let mut t = new_test_ext();
+	with_externalities(&mut t, || {
+		for _ in 0..n {
+			module::Module::<Runtime>::deposit_event(
+				module::Event::Complex(vec![1, 2, 3], 2, 3, 899)
+			);
+		}
+	});
+}
+
+fn sr_system_benchmark(c: &mut Criterion) {
+	c.bench_function("deposit 100 events", |b| {
+		b.iter(|| deposit_events(black_box(100)))
+	});
+}
+
+criterion_group!(benches, sr_system_benchmark);
+criterion_main!(benches);