Rewrote keys with explicit recursion
Rewrite t_foldl'. It was not testing anything.
Rewrite t_unionR using ===
Rewrite t_deleteMin using ===
Rewrite t_deleteMax using ===
Rewrite t_insert_present and t_insert_missing with ===
Differences implemented
Code simplified
Merged with intersection
intersectionWithKey and differenceWithKey unified
Error messages fixed
Merge pull request #53 from kgeorgiy/difference

Implemented intersection and difference methods
Flesh out the Set type a little
Merge pull request #48 from kgeorgiy/keys

Rewrote keys with explicit recursion
Merge pull request #50 from mikesteele81/update-tests

Update tests
Remove trailing whitespace :-(
Remove redundant type witnesses
Simplify t_insertWith_present and t_insertWith_missing
Remove redefinition of Control.Arrow.first.