[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)
In the method just above this you use `#tap` to perform some work, capture the return value, then do something else and return the value. You could use the same pattern here:

```ruby
new(thread).tap do
  thread.run
end
```

Not saying it's better or worse than this code, just consistent with an earlier pattern.