mithril.js: Delegation to `vnode.state` for view methods break `vnode.state` assignment idiom

This was initially brought up by @pygy, but somehow flew right under the radar until recently… 😟

TL;DR: this no longer works, but will instead throw internal errors:

var Component = {
    oninit(vnode) { vnode.state = {counter: 1} }
    view(vnode) {
        return m("div", [
            m("div", ["Count: " + vnode.state.counter]),
            m("button", {onclick() { vnode.state.counter++ }}, "Increment")
        ])
    }
}

m.mount(document.body, m(Component))

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Where @pygy and I changed to using vnode.state instead of vnode.tag as the source of truth for component hooks, that meant the previous idiom of assigning your initial state to vnode.state no longer works now, because the prototype that contains the view and other hooks is now lost.

We’ll have to document this as a breaking change in the changelog for v1.1, and we’ll likely have to edit the documentation accordingly to not write to vnode.state where applicable and to document vnode.state as read-only/volatile (since Mithril sets it on its own).