Use Proxy for CasedObject getters

[replygirl]

Return a Proxy from each getter with something like:

class CasedObject {

  // ...

  public get options() { return this.proxied('_options') }

  // ...

  public get value() { return this.proxied('_value') }

  // ...

  private proxied(p: string) {
    return new Proxy([this[p], {
      get: (_x, k) => this[p][k],
      set: (_x, k, v) => this[p] = { ...this[p], [k]: v }
    }])
  }

  // ...

}