Trac #12944 showed that the DsBinds code that implemented a
SPECIALISE pragma was inadequate if the constraints solving
added let-bindings for dictionaries.  The result was that
we ended up with an unbound dictionary in a DFunUnfolding -- and
Lint didn't even check for that!

Fixing this was not entirely straightforward

* In DsBinds.dsSpec we use a new function
     TcEvidence.collectHsWrapBinders
  to pick off the lambda binders from the HsWapper

* dsWrapper now returns a (CoreExpr -> CoreExpr) function

* CoreUnfold.specUnfolding now takes a (CoreExpr -> CoreExpr)
  function it can use to specialise the unfolding.

On the whole the code is simpler than before.