Set up `style` mutation monitoring

Fixes #2008
This works by storing the style and its copy onto the component, and do the appropriate comparison during `_updateDOMProperties`.
This doesn't work if between two style settings, we set the style to some other random objects. But I think this'll do.

Also, rather than storing styleMutationWarning[ownerName], I guess I could have hashed ownerName + stringified style1 and style2.
I think we want to do this check regardless? like if I do

```
var style = {background: 'red'};
React.render(<div style={style} />, el);
style.background = 'green';
// no rerender with `style`
React.render(<div style={{background: 'green'}} />, el);
```

then we probably currently update to green correctly but will not in the future when we stop cloning.