Make core::future sendable

[brson]

They can be built from sendable pipe endpoints now (instead of unsendable ports), but they still contain boxes. Once futures are sendable they should start being much more useful and composable.

[eholk]

I just poked at this for a bit. Because the task builder includes a notification channel, and a port gets stored somewhere, we basically need to replace these with pipes. This makes task_builder non-copyable (which we want), but we can't do this yet because we don't have moveable self. Thus, I suspect this bug depends on #2585, or at least we have to be willing to get rid of the future_result option in task.rs.

[brson]

Fixed by #3421