Blend.Single changes

[OttoHatt]

• Allow returning nil from the observable. We know that it will never evaluate to anything interesting, so there's no point creating a brio (which are not free). (Should also probably skip creating a brio if the passed observable is complete and evaluates to nil, i.e. == Rx.EMPTY?).
• Propagate state of passed observable. This has just become a slightly more efficient version of a switchMap + toBrio.
• Returning from a subscription does nothing.

[Quenty]

we can pass sub:GetFailComplete() here. Note I think it's fine to complete this here, but a failure could cause other observables in the blend tree to also fail. Most of Nevermore doesn't really leverage the failure state that much.

[Quenty]

If you think it's safe to merge this, I'm down for it.

[OttoHatt]

I wanted the completion, I didn't realise this would could pass failures. I was only vaguely aware subscriptions had this - I know for certain I'm not handling it!

Though it's a breaking change, I would want to propagate this. I think hiding errors is bad. But silently failing is also bad, because I'm not handling this case and wouldn't know why my subscription stopped. I think subscriptions should print a warning (similar to promises) for an unhandled failure state (i.e. no failCallback), and then this should be merged?

[Quenty]

Side note: Rx.EMPTY never completes I believe.

[OttoHatt]

If propagating failure is contentious, is it possible to merge just the first of these commits for now?