Changes in version 0.0.1.9000                      

  - Adds experimental task, stream, cancellation, timeout, and
    composition contracts over nanonext asynchronous primitives.
  - Propagates cancellation through task continuations to their source
    task.
  - Adds cancellation-aware task groups, rho_race(), source-cancelling
    deadlines, global stream collection deadlines, and timeout
    propagation through derived streams.
  - Adds a cancellable serial task queue whose queued entries do not
    cancel active work.