Add simple Identity Map (#73)

* When persisted, model instances are registered with an in-memory
store. When deleted, they are removed from the store and the new `stale`
property becomes `true`.
* Model attributes can now be synced across instances. If assigning an
attribute, that attribute will not be synced until save. After save, all
instances of that Model will sync attributes.
* In order to sync attributes, we listen for updates from the central
store. This means to be garbage collected the listener must be removed.
You can do this by calling `model.unlisten()`. Framework-specific
libraries will ensure `unlisten()` is called automatically when the
framework no longer references the instance.
* When associations return related objects, they remove anything that
was deleted from the store. This way `await book.destroy()` will affect
the `author.books` relationship.

Because the behavior of modifying instances out from under you can be
seen as an implicit (and possibly undesired) side-effect, this behavior
is opt-in:

```ts
// ApplicationRecord
static sync = true
```

Author: richmolj

lib-esm/associations.js

@@ -8,7 +8,7 @@ export declare class NullStorageBackend implements StorageBackend {
     setItem(key: string, value: string | undefined): void;
     removeItem(key: string): void;
 }
-export declare class LocalStorage {
+export declare class CredentialStorage {
     private _jwtKey;
     private _backend;
     constructor(jwtKey: string | false, backend?: StorageBackend);