Multiple storage root support (#902)

* Implement a non generic version of child delta trie

* Use delta_trie_root in state_machine

* Expand InMemory backend to support multi-storage

* Create Consolidate trait

* Fix all crate compile and remove unused OverlayedChanges::drain

* Implement child storage root support and overlay changes

* Add child storage reader

* Add child storage writer

* Implement child storage cleaning

* Fix light backend compile

* Add all required ext functions for wasm executor

* Add ext def to io

* Add all io functions

* Fix nostd compile

* Add simple test

* Remove unnecessary vec copy in child_storage_root_transaction

* Use values_mut/for_each to make it shorter

* Use extend to shorter a for loop

* Move record_all_keys to trie so it's easier to generic them later

* space -> tab

* Remove to_owned in debug format

* Clean out all to_owned

* Break debug_trace to multiple lines

* Remove 0..

* UserError copy/paste typo

* Replace Vec::from_raw_parts by slice::from_raw_parts

* Use iter::empty()

* Wrap some long lines

* Wrap a missing line

* Remove unnecessary map

https://github.com/paritytech/substrate/pull/856#discussion_r226222663

* Call ext_free after from_raw_parts

* Fix tests in other crates

substrate/core/primitives/src/storage.rs

@@ -72,4 +72,13 @@ pub mod well_known_keys {
 
 	/// Changes trie configuration is stored under this key.
 	pub const CHANGES_TRIE_CONFIG: &'static [u8] = b":changes_trie";
+
+	/// Prefix of child storage keys.
+	pub const CHILD_STORAGE_KEY_PREFIX: &'static [u8] = b":child_storage:";
+
+	/// Whether a key is a child storage key.
+	pub fn is_child_storage_key(key: &[u8]) -> bool {
+		key.starts_with(CHILD_STORAGE_KEY_PREFIX)
+	}
+
 }