* 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.