Note [Deep skolemisation]

deeplySkolemise decomposes and skolemises a type, returning a type
with all its arrows visible (ie not buried under foralls)

Examples:

  deeplySkolemise (Int -> forall a. Ord a => blah)
    =  ( wp, [a], [d:Ord a], Int -> blah )
    where wp = \x:Int. /\a. \(d:Ord a). <hole> x

  deeplySkolemise  (forall a. Ord a => Maybe a -> forall b. Eq b => blah)
    =  ( wp, [a,b], [d1:Ord a,d2:Eq b], Maybe a -> blah )
    where wp = /\a.\(d1:Ord a).\(x:Maybe a)./\b.\(d2:Ord b). <hole> x

In general,
  if      deeplySkolemise ty = (wrap, tvs, evs, rho)
    and   e :: rho
  then    wrap e :: ty
    and   'wrap' binds tvs, evs

ToDo: this eta-abstraction plays fast and loose with termination,
      because it can introduce extra lambdas.  Maybe add a `seq` to
      fix this