After rumination, yet again, on the subject of arity calculation,
I have redone what an ArityType is (it's purely internal to the
CoreArity module), and documented it better.  The result should
fix #3959, and I hope the related #3961, #3983.

There is lots of new documentation: in particular
 * Note [ArityType]  
   describes the new datatype for arity info

 * Note [State hack and bottoming functions] 
   says how bottoming functions are dealt with, particularly
   covering catch# and Trac #3959

I also found I had to be careful not to eta-expand single-method
class constructors; see Note [Newtype classes and eta expansion].
I think this part could be done better, but it works ok.