Light client friendly events (#2491)

* Sketch of indexed events.

* Get EventIndex by holding another variable.

* Add some docs.

* Use DoubleMap to store reverse topic index

* Implement StorageDoubleMap::append

* Use append for EventTopics.

* Refactor.

* Avoid `mutate`

* Docs.

* Add topics to EventRecord

* Update tests.

* Rebuild.

* Bump version.

* Event topics test.

* Mix in BlockNumber to distinguish updates

* Fix srml-system test.

* Post merge fixes.

* Comments/TODO.

substrate/srml/support/src/storage/mod.rs

@@ -401,6 +401,20 @@ pub trait StorageDoubleMap<K1: Codec, K2: Codec, V: Codec> {
 		KArg1: Borrow<K1>,
 		KArg2: Borrow<K2>,
 		F: FnOnce(&mut Self::Query) -> R;
+
+	/// Append the given items to the value under the key specified.
+	///
+	/// `V` is required to implement `codec::EncodeAppend<Item=I>`.
+	fn append<KArg1, KArg2, I>(
+		k1: KArg1,
+		k2: KArg2,
+		items: &[I],
+	) -> Result<(), &'static str>
+	where
+		KArg1: Borrow<K1>,
+		KArg2: Borrow<K2>,
+		I: codec::Encode,
+		V: EncodeAppend<Item=I>;
 }
 
 impl<K1: Codec, K2: Codec, V: Codec, U> StorageDoubleMap<K1, K2, V> for U
@@ -453,6 +467,20 @@ where
 	{
 		U::mutate(k1.borrow(), k2.borrow(), f, &RuntimeStorage)
 	}
+
+	fn append<KArg1, KArg2, I>(
+		k1: KArg1,
+		k2: KArg2,
+		items: &[I],
+	) -> Result<(), &'static str>
+	where
+		KArg1: Borrow<K1>,
+		KArg2: Borrow<K2>,
+		I: codec::Encode,
+		V: EncodeAppend<Item=I>,
+	{
+		U::append(k1.borrow(), k2.borrow(), items, &RuntimeStorage)
+	}
 }
 
 /// child storage NOTE could replace unhashed by having only one kind of storage (root being null storage