Add a scheduler type to the task API

I've resisted having first class schedulers in the task API, but sometimes you do want to spawn functions onto schedulers that you aren't running on.

```
impl Scheduler {
  fn task() -> TaskBuilder;
}
```

Add a way to get the current scheduler. Add a way to create new schedulers. Remove the code from task builder that deals with schedulers - if you want a scheduler you make one.
