Move foldlWithKeyWith, foldrWithKeyWith, foldlWithKey, foldlWithKey', foldrWithKey, and foldrWithKey' to Internal module.
Change Foldable imports for Internal module.
Rewrite foldl' and foldr' in terms of foldlWithKey' and foldrWithKey' respectively.
Define foldl and foldr for Foldable instance of Node.
Rewrite foldl and foldr in terms of Foldable instance.
Rewrite elems in terms of foldr.
Remove duplicate copy of minKey
Merge