Child storage tests and genesis fix. (#3185)

* Using child storage, (srml-support only), test failing .

* fix simple tests.

* Enumerable by requiring owned struct (previous form only allow
&'static).
Broken tests are from genesis init.

* implement for_child_keys_with_prefix

* indent

* clear_child_prefix fix.

* clear_child_prefix fix 2.

* fix for storage_impl, if/when allowing child and not child this could be
reverted.

* Fix lot of urlinked child genesis, still need to look upon actual
genesis srml module code.
Probably still a lot of broken code needing debugging.

* switch well_known_key to their associated module child trie.
Fix a genesis init (balance).
Complete some testing.
Comment some tests before using.

* fixing test runtime child keys

* latest commit fix broken genesis init

* fix system balances child name.

* Important fix: storage_root from test externalities need children (it is
already the case for ext).

* executive root with child calculation

* Avoid empty trie on test ext.

* Symetric removal of key for system.

* commenting changes related tests.

* Remove child module specifics.

* fix issues.

* fix some formatting

* fix bench and bump runtime

* Remove extend_storage_overlays, assimilate_storage do the same as is
proper considering srml macro.

* Fix warning for assimilate.

* Removing kill as they do not impact any test cases.

* Use tuple of storage map instead of two parameters. This changes the
behavior of decl_storage genesis build closure (breaking api).

* Do not use build storage before assimilate.

* fix error

* Update core/state-machine/src/backend.rs

substrate/core/client/src/light/backend.rs

@@ -381,6 +381,15 @@ where
 		// whole state is not available on light node
 	}
 
+	fn for_child_keys_with_prefix<A: FnMut(&[u8])>(
+		&self,
+		_storage_key: &[u8],
+		_prefix: &[u8],
+		_action: A,
+	) {
+		// whole state is not available on light node
+	}
+
 	fn storage_root<I>(&self, _delta: I) -> (H::Out, Self::Transaction)
 	where
 		I: IntoIterator<Item=(Vec<u8>, Option<Vec<u8>>)>
@@ -456,6 +465,20 @@ where
 		}
 	}
 
+	fn for_child_keys_with_prefix<A: FnMut(&[u8])>(
+		&self,
+		storage_key: &[u8],
+		prefix: &[u8],
+		action: A,
+	) {
+		match *self {
+			OnDemandOrGenesisState::OnDemand(ref state) =>
+				StateBackend::<H>::for_child_keys_with_prefix(state, storage_key, prefix, action),
+			OnDemandOrGenesisState::Genesis(ref state) =>
+				state.for_child_keys_with_prefix(storage_key, prefix, action),
+		}
+	}
+
 	fn storage_root<I>(&self, delta: I) -> (H::Out, Self::Transaction)
 	where
 		I: IntoIterator<Item=(Vec<u8>, Option<Vec<u8>>)>