Deprecate `computed()` + `task()`

Async computed should be designed much better that we have right now in computed() + task().

For instance, we need to cover error case, have explicit loading state and a solution to mark loading state between values.

So the plan:

1. Create a better API
2. Mark computed() + task() as deprecated with warning
3. Do a N+1 major release
4. Remove computed() + task() in N+2 major release.