- BLACKHOLE_BQ is a mutable object, because new threads get added to
  its blocking_queue field.  Hence add a mut_link field and treat it
  as mutable in the garbage collector.

- Change StgBlackHole to StgBlockingQueue while I'm at it.

- Optimise evacuation of black holes: don't copy the padding
  words, just skip over them.

- Several garbage collection fixes.

- Improve sanity checking: now the older generations are fully checked
  at each GC.