gizmo: support cleanup

[unional]

justland/just-web#21

const gizmo = define({
  async create() {
    return [{}, () => () => 'clean up']
  }
})

How to trigger? This will be breaking change:

const [app, cleanup] = await incubate().with(...).create()

cleanup()

One solution is to use symbol:

const app = await incubate().with(...).create()

await cleanup(app)

async function cleanup(app) {
  app[destructorSymbol]()
}

or use the same pattern:

const app = await incubate().with(...).create(() => () => clean up)

This design conflict with #161.
Need to find to way to make both of them work.