default ppr method for CmmGraph now tells more about the representation

default ppr method for CmmGraph now tells more about the representation
(Previously, ppr had tried to make the zipper representation look as
much like the ListGraph representation as possible.  This decision was
unhelpful for debugging, so although the old code has been retained,
the new default is to tell it like it is.  It may be possible to
retire PprCmmZ one day, although it may be desirable to retain it as
the internal form becomes less readable.