EnumerableStorageMap (#1763)

* Refactor decl_storage a bit to allow easier impl of linked map.

* A bunch of refactorings for storage generation.

- Rename StorageMap and ChildrenStorageMap to avoid confusion with generator::StorageMap.
- Separate implementation from the procedural macro code to clean it up.
- Make sure that genesis is initialised using the `StorageValue/StorageMap`
  generated implementations instead of going RAW.

* WiP: Writing test.

* Basic implementation.

* Implement enumeration.

* Fix non-std issues.

* fix warning

* Fix test-client.

* Address review grumbles - part 1

* Avoid cloning the key, relax Storage requirements.

* Rebuild runtime.

* Remove dangling todo.

substrate/core/client/src/backend.rs

@@ -18,7 +18,7 @@
 
 use crate::error;
 use primitives::ChangesTrieConfiguration;
-use runtime_primitives::{generic::BlockId, Justification, StorageMap, ChildrenStorageMap};
+use runtime_primitives::{generic::BlockId, Justification, StorageOverlay, ChildrenStorageOverlay};
 use runtime_primitives::traits::{AuthorityIdFor, Block as BlockT, NumberFor};
 use state_machine::backend::Backend as StateBackend;
 use state_machine::ChangesTrieStorage as StateChangesTrieStorage;
@@ -79,7 +79,7 @@ pub trait BlockImportOperation<Block, H> where
 	/// Inject storage data into the database.
 	fn update_db_storage(&mut self, update: <Self::State as StateBackend<H>>::Transaction) -> error::Result<()>;
 	/// Inject storage data into the database replacing any existing data.
-	fn reset_storage(&mut self, top: StorageMap, children: ChildrenStorageMap) -> error::Result<H::Out>;
+	fn reset_storage(&mut self, top: StorageOverlay, children: ChildrenStorageOverlay) -> error::Result<H::Out>;
 	/// Set top level storage changes.
 	fn update_storage(&mut self, update: Vec<(Vec<u8>, Option<Vec<u8>>)>) -> error::Result<()>;
 	/// Inject changes trie data into the database.