Add foldr1 and `foldl1` members to Foldable instance

[JordanMartinez]

This repo now fails to build due to the changes made in foldable-traversable. See https://travis-ci.org/github/purescript/purescript-unfoldable/builds/735873107#L722

  The following type class members have not been implemented:

  foldr1 :: forall a0. (a0 -> a0 -> a0) -> Tuple a a0 -> a0

  foldl1 :: forall a0. (a0 -> a0 -> a0) -> Tuple a a0 -> a0