CPR control

1.  Remove -fno-cpr, add -fcpr-off which is a simple static flag
    for switching the new CPR analysis off altogether.
    (The "-fno" machinery is rather complicated.)

2.  Rejig SimplCore a little so that the "old strictness analyser"
    runs both the old strictness analyser and the old CPR analyser,
    which makes it more like the new strictness/CPR analyser.

    (How much longer we keep the old strictness/CPR analyser in the
    compiler at all I don't know.  It's just for comparision purposes
    when we write the paper.)