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.
fix#1740
Previous fix for #1727 caused `watchEffect` to also recursively trigger
itself on reactive array mutations which implicitly registers array
`.length` as dependencies and mutates it at the same time.
This fix limits recursive trigger behavior to only `watch()` callbacks
since code inside the callback do not register dependencies and
mutations are always explicitly intended.
