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.
Implemented `mapMaybe`
Implemented `mapEither`
Remove redundant brackets.
Substitute lambdas with equivalent 'const'.
Resolve minKey shadowing warning.
Implement general function insertLookupGen.

Implement insertLookupWithKey in terms of insertLookupGen.
Loosen forcePair type.
Rewrite insertWithKey in terms of insertLookupGen.
Rewrite insert in terms of insertLookupGen.
Rewrite insertWith in terms of insertLookupGen.
Remove duplicate copy of minKey
Merge
Merge pull request #64 from archblob/master

A little cleaning.
A little bit of const golfing
Merge pull request #70 from kgeorgiy/mapMaybe

Implemented `mapMaybe` and `mapEither`