Add a scheduler type to the task API

[brson]

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.

[catamorphism]

Still an issue, as far as I know.

[brson]

Obsolete in the new scheduler.