List-monomorphic `foldr'`
Per CLC discussion, add list-monomorphic foldr'. The commit message is:
While a strict (i.e. constant space) right-fold on lists is not
possible, the default foldr' is optimised for structures like
Seq, that support efficient access to the right-most elements.
The original default implementation seems to have a better
constant factor for lists, so we add a monomorphic implementation
in GHC.List.
Should this be re-exported from Data.List? That would be a
user-visible change if both Data.Foldable and Data.List are
imported unqualified...