Allow a Durable Object to create new unique ids and get ids from a string

[jacwright]

In a durable object, the proxies overwrite the stubs on state. This will allow a durable object to still be able to create new ids and convert id strings to id objects for use in Proxy.get(id).

const stub = this.state.env.Counter;
const newId = stub.newUniqueId();
const idObj = stub.idFromString(storedObjectId);
this.state.Counter.get(newId).increment();
this.state.Counter.get(idObj).increment();

This is needed for creating fan-out systems or connection pooling like https://github.com/cloudflare/dog/.

[kwhitley]

No objections here!