Ability to specify cost value for computations

Some functions may require more resources than others so the ability to set the function cost is useful to avoid overloading workers.

Imagine function we have 1 worker with 4 threads and `F(x)`, `G(x)` while 4 instances of `G(x)` could be computed easily on the worker, 2 or more instances of `F(x)` will overload the worker CPU thus a possible API could be.

```python
f.submit(x=123, compute_cost=4)
```
This will allow the function to fill up the 4 threads that the worker has without submitting more tasks to it before completing this function.

Of course, the API requires considerable design choices but this issue serves as a discussion starter.