Make the specialiser handle polymorphic specialisation

Ticket #13873 unexpectedly showed that a SPECIALISE pragma made a
program run (a lot) slower, because less specialisation took place
overall. It turned out that the specialiser was missing opportunities
because of quantified type variables.

It was quite easy to fix. The story is given in
    Note [Specialising polymorphic dictionaries]

Two other minor fixes in the specialiser

* There is no benefit in specialising data constructor /wrappers/.
  (They can appear overloaded because they are given a dictionary
  to store in the constructor.)  Small guard in canSpecImport.

* There was a buglet in the UnspecArg case of specHeader, in the
  case where there is a dead binder. We need a LitRubbish filler
  for the specUnfolding stuff.  I expanded
  Note [Drop dead args from specialisations] to explain.

There is a 4% increase in compile time for T13056, because we generate
more specialised code.  This seems OK.

Metric Increase:
    T13056