Remove support for vnode.state = ...

- Remove docs for it
- Remove tests for it
- Add runtime check for unexpected reassignment, since we can't freeze
  the property (we internally have to be able to modify it)

render/vnode.js

@@ -1,7 +1,7 @@
 "use strict"
 
 function Vnode(tag, key, attrs, children, text, dom) {
-	return {tag: tag, key: key, attrs: attrs, children: children, text: text, dom: dom, domSize: undefined, state: undefined, _state: undefined, events: undefined, instance: undefined, skip: false}
+	return {tag: tag, key: key, attrs: attrs, children: children, text: text, dom: dom, domSize: undefined, state: undefined, events: undefined, instance: undefined, skip: false}
 }
 Vnode.normalize = function(node) {
 	if (Array.isArray(node)) return Vnode("[", undefined, undefined, Vnode.normalizeChildren(node), undefined, undefined)