Improve data variety for data-driven tests (immutable, vs mutable, vs thread-yield, vs hot-delayed) plus massive refactoring

[abelbraaksma]

Basically as in the title. We now have these sequences, each returning either an empty sequence variant, or a sequence 1..10, either with mutable data on each enumeration, using byref data, plain old mutable, or let bindings, using spin delays or thread-yielding delays, or deeply nested taskSeq.

These types work with Theory and ClassData. Just pass it this class name:

    type TestEmptyVariants() as this =
        inherit TheoryData<EmptyVariant>()

        do
            this.Add EmptyVariant.CallEmpty
            this.Add EmptyVariant.Do
            this.Add EmptyVariant.DoBang
            this.Add EmptyVariant.YieldBang
            this.Add EmptyVariant.YieldBangNested
            this.Add EmptyVariant.DelayDoBang
            this.Add EmptyVariant.DelayYieldBang
            this.Add EmptyVariant.DelayYieldBangNested

    type TestImmTaskSeq() as this =
        inherit TheoryData<SeqImmutable>()

        do
            this.Add SeqImmutable.Sequential_YieldBang
            this.Add SeqImmutable.Sequential_Yield
            this.Add SeqImmutable.Sequential_For
            this.Add SeqImmutable.Sequential_Combine
            this.Add SeqImmutable.Sequential_Zero
            this.Add SeqImmutable.ThreadSpinWait
            this.Add SeqImmutable.AsyncYielded
            this.Add SeqImmutable.AsyncYielded_Nested

    type TestSideEffectTaskSeq() as this =
        inherit TheoryData<SeqWithSideEffect>()

        do
            this.Add SeqWithSideEffect.Sequential_YieldBang
            this.Add SeqWithSideEffect.Sequential_Yield
            this.Add SeqWithSideEffect.Sequential_For
            this.Add SeqWithSideEffect.Sequential_Combine
            this.Add SeqWithSideEffect.Sequential_Zero
            this.Add SeqWithSideEffect.ThreadSpinWait
            this.Add SeqWithSideEffect.AsyncYielded
            this.Add SeqWithSideEffect.AsyncYielded_Nested

[abelbraaksma]

Let's get this in, we'll apply these changes to the other tests soon(ish)