Now uses MutationObserver to ensure it works even for HOCs

fix #3894

fix #4129

BREAKING CHANGE: Output of SFC using `<style scoped>` generated by 3.2+
will be incompatible w/ runtime <3.2.

Also allows render function usage like the following:

```js
h({
  '.prop': 1, // force set as property
  '^attr': 'foo' // force set as attribute
})
```

Based on #2345 , but with smaller API change

- Use class implementation for `ReactiveEffect`
- Switch internal creation of effects to use the class constructor
- Avoid options object allocation
- Avoid creating bound effect runner function (used in schedulers) when not necessary.
- Consumes ~17% less memory compared to last commit
- Introduces a very minor breaking change: the `scheduler` option passed to `effect` no longer receives the runner function.