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/sr-io/src/lib.rs

@@ -141,6 +141,9 @@ export_api! {
 		/// Clear the storage entries with a key that starts with the given prefix.
 		fn clear_prefix(prefix: &[u8]);
 
+		/// Clear the child storage entries with a key that starts with the given prefix.
+		fn clear_child_prefix(storage_key: &[u8], prefix: &[u8]);
+
 		/// "Commit" all existing operations and compute the resultant storage root.
 		fn storage_root() -> [u8; 32];
 
@@ -401,7 +404,7 @@ mod imp {
 
 #[cfg(feature = "std")]
 pub use self::imp::{
-	StorageOverlay, ChildrenStorageOverlay, with_storage, with_storage_and_children,
+	StorageOverlay, ChildrenStorageOverlay, with_storage,
 	with_externalities
 };
 #[cfg(not(feature = "std"))]