Work stealing for sparks

Work stealing for sparks

   Spark stealing support for PARALLEL_HASKELL and THREADED_RTS versions of the RTS.

  Spark pools are per capability, separately allocated and held in the Capability
  structure. The implementation uses Double-Ended Queues (deque) and cas-protected
  access.

  The write end of the queue (position bottom) can only be used with
  mutual exclusion, i.e. by exactly one caller at a time.
  Multiple readers can steal()/findSpark() from the read end
  (position top), and are synchronised without a lock, based on a cas
  of the top position. One reader wins, the others return NULL for a
  failure.

  Work stealing is called when Capabilities find no other work (inside yieldCapability),
  and tries all capabilities 0..n-1 twice, unless a theft succeeds.

  Inside schedulePushWork, all considered cap.s (those which were idle and could
  be grabbed) are woken up. Future versions should wake up capabilities immediately when
  putting a new spark in the local pool, from newSpark().

Patch has been re-recorded due to conflicting bugfixes in the sparks.c, also fixing a
(strange) conflict in the scheduler.