See Note [DFun unfoldings] in CoreSyn.  The issue here is that 
you can't tell how many dictionary arguments a DFun needs just
from looking at the Arity of the DFun Id: if the dictionary is
represented by a newtype the arity might include the dictionary
and value arguments of the (single) method.

So we need to record the number of arguments need by the DFun
in the DFunUnfolding itself.  Details in 
   Note [DFun unfoldings] in CoreSyn