* In stg_ap_0_fast, if we're evaluating a thunk, the thunk might
  evaluate to a function in which case we may have to adjust its CCS.

* The interpreter has its own implementation of stg_ap_0_fast, so we
  have to do the same shenanigans with creating empty PAPs and copying
  PAPs there.

* GHCi creates Cost Centres as children of CCS_MAIN, which enterFunCCS()
  wrongly assumed to imply that they were CAFs.  Now we use the is_caf
  flag for this, which we have to correctly initialise when we create a
  Cost Centre in GHCi.