Add willChangeFromState to views-dom

There should be a common way to set `willChange$` from a `state$`.  If #235 doesn't happen, we should have something like this:

```typescript
function willChangeFromState(motionState$, activeName = 'transform')
```

If #235 does, this effectively becomes `rewriteWhenTrue`.

(See also #240 for the opacity:pointerEvents equivalent)