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/node/runtime/src/lib.rs

@@ -60,7 +60,7 @@ pub const VERSION: RuntimeVersion = RuntimeVersion {
 	impl_name: create_runtime_str!("substrate-node"),
 	authoring_version: 10,
 	spec_version: 62,
-	impl_version: 62,
+	impl_version: 63,
 	apis: RUNTIME_API_VERSIONS,
 };