PortalGun: Dest accepts render callback

[brigand]

Dest should accept a render callback which is always called when the Dest is re-rendered, or there's an update at a Source. The main use cases here are transitions, and wrapping the node in another element.

<Dest />

// same as...

<Dest>
  {(nodeOrNull) => nodeOrNull}
</Dest>

For transitions, it would be compatible with react-transition-group, without us doing anything special.

The general wrapper case allows conditionally rendering a wrapper, based on the node existing or not.

<Dest>
  {(nodeOrNull) => nodeOrNull
    ? <Modal>{nodeOrNull}</Modal>
    : null
  }
</Dest>