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/srml/generic-asset/src/lib.rs

@@ -479,12 +479,11 @@ decl_storage! {
 		config(endowed_accounts): Vec<T::AccountId>;
 
 		build(|
-			storage: &mut sr_primitives::StorageOverlay,
-			_: &mut sr_primitives::ChildrenStorageOverlay,
+			storage: &mut (sr_primitives::StorageOverlay, sr_primitives::ChildrenStorageOverlay),
 			config: &GenesisConfig<T>| {
 			config.assets.iter().for_each(|asset_id| {
 				config.endowed_accounts.iter().for_each(|account_id| {
-					storage.insert(
+					storage.0.insert(
 						<FreeBalance<T>>::key_for(asset_id, account_id),
 						<T::Balance as codec::Encode>::encode(&config.initial_balance)
 					);