Enhance type definitions for Executor

[spawnia]

I converted Executor and ReferenceExecutor to use native type hints where possible and cleaned up a few mismatched types. Turns out a few of them, if translated 1-1, caused type errors.

Now we can be sure the types are correct and will have an easier time when trying to understand what is happening in the code and when refactoring.

[coveralls]

Coverage decreased (-0.1%) to 86.204% when pulling 79d5c74 on spawnia:executor-types into 4f34309 on webonyx:master.

[shmax]

Pretty keen! 👍

No changes to the baseline file..?

[vladar]

Great stuff, thanks! 👍