(this is number 7 of 9 commits to be applied together)

  The code generator now incorporates the update avoidance
  optimisation: a thunk of __o type is now made SingleEntry rather
  than Updatable.

  We want to verify that SingleEntry thunks are indeed entered at most
  once.  In order to do this, -ticky turns on eager blackholing.
  Ordinary thunks will be dealt with by the RTS, but CAFs are
  blackholed by the code generator.  We blackhole with new blackholes:
  SE_CAF_BLACKHOLE.  We will enter one of these if we attempt to enter
  a SingleEntry thunk twice.