Nat Goodspeed:
- Change some doc references from 'algorithm' to 'sched_algorithm'.
- Initial cut at supporting arbitrary user-coded scheduler properties.
- Set fiber_properties::sched_algo_ every time through awakened().
- Define sched_algorithm methods on fiber_base*, not worker_fiber*.
- Simplify detail::fifo by making tail_ point to last link pointer.
- Reimplement waiting_queue::push() using pointer-to-pointer trick.
- Reimplement waiting_queue::move_to() using fiber_base** scan.
- Make bounded_queue::tail_ a ptr* to simplify appending new nodes.
- Make unbounded_queue::tail_ a ptr* to simplify linking new nodes.
- Remove thread_affinity flag and access methods.
- Re-add thread_affinity specific to workstealing_round_robin.
- Remove 'priority' for every fiber, and its support methods.