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.