Add silent superclass parameters (again)silent-sc-args

Silent superclass parameters solve the problem that
the superclasses of a dicionary construction can easily
turn out to be (wrongly) bottom.  The problem and solution
are described in
   Note [Silent superclass arguments] in TcInstDcls

I first implemented this fix (with Dimitrios) in Dec 2010, but removed
it again in Jun 2011 becuase we thought it wasn't necessary any
more. (The reason we thought it wasn't necessary is that we'd stopped
generating derived superclass constraints for *wanteds*.  But we were
wrong; that didn't solve the superclass-loop problem.)

So we have to re-implement it.  It's not hard.  Main features:

  * The IdDetails for a DFunId says how many silent arguments it has

  * A DFunUnfolding describes which dictionary args are
    just parameters (DFunLamArg) and which are a function to apply
    to the parameters (DFunPolyArg).  This adds the DFunArg type
    to CoreSyn

  * Consequential changes to IfaceSyn.  (Binary hi file format changes
    slightly.)

  * TcInstDcls changes to generate the right dfuns

  * CoreSubst.exprIsConApp_maybe handles the new DFunUnfolding

The thing taht is *not* done yet is to alter the vectoriser to
pass the relevant extra argument when building a PA dictionary.

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