Fix Trac #3406 (albeit not very satisfactorily): scoped type variables

Fix Trac #3406 (albeit not very satisfactorily): scoped type variables

The issue here is this:

  type ItemColID a b = Int  -- Discards a,b

  get :: ItemColID a b -> a -> ItemColID a b
  get (x :: ItemColID a b) = x :: ItemColID a b

The pattern signature for 'x' doesn't actually rigidly bind a,b.
This crashed GHC 6.10 with a 'readFilledBox' panic.  Now we fail
with an erroe message

With the new outside-in algorithm we'll be able to accept this program.