async generators wip

[heapwolf]

This PR allows the render function to be async or async *, for example...

class FooBar extends Tonic {
  async * render () {
    yield "Loading..."
    yield await getData()
  }
}

However, it is not yet possible to test correctly since @juliangruber's tape-run module does not support a current browser.

cc @davidmarkclements

[heapwolf]

I think this could be published as @conductorlab/tonic@next and then once the tests are passing again it could be published normally.