Implement client side data structures

Client side data structures is a feature which allows you to manage and mutate a data structure on the client without storing all the data on the server.

An example of a managed data structure would be the `ClientList`. The `ClientList` manages a list on the client, with explicit instructions of mutations being sent from the server to the client.

`ClientList` is a superset of the mechanism that `phx-update` and temporary assigns provides in LiveView, and could be used to implement that mechanism.

Some imagined managed data structures:
* `ClientList` - A list managed on the client. Mutations are explicitly generated, the whole list is only ever stored on the client.
* `ClientMap` - Same as `ClientList`, but the data structure on the client is a map instead.
* `ClientBinary`/`ClientString`