Add DeferredCurrentScheduler
Add Enumerable.delay and Observable.delay
Add Enumerable.deliverOn and Observable.deliverOn
Meh, just remove the Stream class after all

It’s of dubious value, given how many operator implementations are
ending up in the subclasses anyways.

Removing it might lead to simpler implementations of the aforementioned
as well (because they’re not hacking around the mismatch between
Enumerable and Observable semantics).