Refactor TaskState.Pending to be Optional<TaskState>.None

[richardjrossiii]

This allows better guarantees for operations that take a TaskState, such
as being unable to set the state of a task to .Pending anymore.

Depends on #25

[nlutsenko]

Actually, I don't think I agree with this approach.
The fact that we have Pending as a case for state is an implementation detail, and if you are setting the task to that state internally - you are making a mistake regardless.
You are adding an extra layer of complexity here, where a Pending state is designated with state being set to nil, whilst the finished state is designated with a concrete value.

I can accept it, but let's talk more on why we want this, since it adds complexity for no explicit value.