Add a "forall" quantifier before rank-n-types

I propose to add "forall" in one place in haskeline. This improves consistency (the other data member `evalTerm` also uses explicit forall). It is planned that future GHC will require "forall" in all polymorphic fields, including those which use => arrow. For more: https://ghc.haskell.org/trac/ghc/ticket/4426. I'm open to any questions.