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/test-client/src/lib.rs

@@ -35,7 +35,7 @@ pub use consensus;
 
 use std::sync::Arc;
 use primitives::Blake2Hasher;
-use runtime_primitives::StorageMap;
+use runtime_primitives::StorageOverlay;
 use runtime_primitives::traits::{Block as BlockT, Header as HeaderT, Hash as HashT};
 use runtime::genesismap::{GenesisConfig, additional_storage_with_genesis};
 use keyring::Keyring;
@@ -126,7 +126,7 @@ fn genesis_config(support_changes_trie: bool) -> GenesisConfig {
 	], 1000)
 }
 
-fn genesis_storage(support_changes_trie: bool) -> StorageMap {
+fn genesis_storage(support_changes_trie: bool) -> StorageOverlay {
 	let mut storage = genesis_config(support_changes_trie).genesis_map();
 	let state_root = <<<runtime::Block as BlockT>::Header as HeaderT>::Hashing as HashT>::trie_root(storage.clone().into_iter());
 	let block: runtime::Block = client::genesis::construct_genesis_block(state_root);