Allow tasks to return values

[eholk]

Right now tasks are all void. If we could return values from tasks, it would make the "go do this asynchronously and get back to me" pattern a lot easier. It would require changes to the type system though (task becomes task[T]), but most of the plumbing is in place to return a value from a task.

[eholk]

I'm not sure this still fits in with the overall view of the task system. We can provide this behavior through wrapper libraries if we really need to.