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/srml/grandpa/src/lib.rs

@@ -136,7 +136,7 @@ decl_storage! {
 	add_extra_genesis {
 		config(authorities): Vec<(T::SessionKey, u64)>;
 
-		build(|storage: &mut primitives::StorageMap, _: &mut primitives::ChildrenStorageMap, config: &GenesisConfig<T>| {
+		build(|storage: &mut primitives::StorageOverlay, _: &mut primitives::ChildrenStorageOverlay, config: &GenesisConfig<T>| {
 			use codec::{Encode, KeyedVec};
 
 			let auth_count = config.authorities.len() as u32;