[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)
Does `Undefined` need to be a module? Would it be more lightweight to use `Undefined = BasicObject.new` or something?