[WIP] Use actor based parallelization

* Fixes longstanding issues with nested `parrallel` execution. (good thing)
* More responsive UI as progress reports happen in main thread with time shedule and not per mutation state event callback. (good thing)
* Allows worker self registration (groundwork for distributed killing)
* Performance is on-par with `parallel`. (good thing)
* Removes frame throttle logic from reporters (good thing).
* No need to make `Mutant::Mutation` instances serializable, reduces building partial result objects to make them transmittable over `parallel`'s interfaces. (good thing)
* More code. (bad thing)
* 100% line coverage (I know I know, its not MT covered fully right now) but better than master ;)
@dkubb There is a very uneasy to spot (and spec) side effect of the code.

```
actor = nil # introduces actor lvar
Thread.start do
  # In current design the actor object needs a reference to the actors thread
  # And it must be visible to the parent thread. For this reason it must be created in one thread
  # and somehow being passed to the other. At this point of the execution its not *sure* the
  # parent thread got scheduled already to create the actor hence we need to suspend the    
  # current thread if not.
  Thread.stop unless actor
  yield actor # after thread got waken
end

# write actor lvar 
actor = new(thread)
thread.start # allow child to access actor
actor # return actor from spawn
```

The transformation to `Object#tap` is not possible without violating this very implicit protocol I designed here initially. Performing the lvar assignment from inside the `Object#tap` block is possible but is even more confusing IMO.

I'm gonna refactor this whole trickery away as it is also very uneasy to spec unless invoking real threads that does not make any sense from unit specs.

The initial code I wrote only had the target to get me started and verify the downstream concepts. I'll now clean the head up.

Your code flag made me aware about that very imprecise protocol I implemented here, and I blindly agreed initially to your change and broke that protocol. Another sign it should not be merged to master as is.