Smarter `Monoid` instance for `ReactElement`

[natefaubion]

React still sometimes has trouble with [] as mempty if you are returning it from a render method. We should use null as mempty. Additionally, we should make a smarter append that does not build nested fragments, but instead concatenates Arrays.

[thomashoneyman]

I think we should try and get this in for the upcoming breaking release for PureScript 0.14.

[thomashoneyman]

We may not be able to get this in for 0.14 due to a backlog of other work, so I'm going to remove the label.

[JordanMartinez]

Fixed by #187

[natefaubion]

To be clear, I only changed it so that mempty is null and it avoids creating redundant fragments if one of the elements is also null. There isn't a reliable way of always flattening to an Array in append, and it may not even be desirable. I think the most reliable way if it's something you want is to use reacts Children.toArray API to flatten at the end.