fix replace stateful widget with vnode

The following test fails:

```js
test("Can replace stateful widget with vnode", function (assert) {
    var statefulWidget  = {
        init: function () {
            return render(h("div.widget"))
        },
        update: function () {},
        destroy: function () {},
        type: "Widget"
    }
 
    var leftNode = h("div", statefulWidget)
    var rightNode = h("div", h("div.vnode"))
 
    var rootNode = render(leftNode)
 
    assert.equal(rootNode.childNodes.length, 1)
    assert.equal(rootNode.childNodes[0].className, 'widget')
 
    var patches = diff(leftNode, rightNode)
    var newRoot = patch(rootNode, patches)
 
    assert.equal(newRoot, rootNode)
 
    assert.equal(newRoot.childNodes.length, 1)
    assert.equal(newRoot.childNodes[0].className, 'vnode')
 
    assert.end()
})
```

I've managed to fix by adding a new kind of patch: `VPatch.DESTROY`

All tests pass, hopefully good to go. As long as my approach seems ok :)