This patch finally makes it possible to have explicit
foralls in an instance decl
   instance forall (a :: *). Eq a => Eq [a] where ...

This is useful to allow kind signatures or indeed
explicicit kind for-alls; see Trac #11519

I thought it would be really easy, because an instance
declaration already contains an actual HsSigType, so all
the syntactic baggage is there.  But in fact it turned
out that instance declarations were kind-checked a
little differently, because the body kind of the forall
is 'Constraint' rather than '*'.

So I fixed that.  There a slight kludge
(see Note [Body kind of a HsQualTy], but it's still a
significant improvement.

I also did the usual other round of refactoring,
improved a few error messages, tidied up comments etc.
The only significant aspect of all that was

  * Kill mkNakedSpecSigmaTy, mkNakedPhiTy, mkNakedFunTy
    These function names suggest that they do something
    complicated, but acutally they do nothing. So I
    killed them.

  * Swap the arg order of mkNamedBinder, just so that it is
    convenient to say 'map (mkNamedBinder Invisible) tvs'

  * I had to improve isPredTy, to deal with (illegal)
    types like
       (Eq a => Eq [a]) => blah
    See Note [isPeredTy complications] in Type.hs

Still to come: user manual documentation for the
instance-decl change.