Implement filter and filterWithKey.
Merge pull request #17 from RamsesDeNorre/filter

Implement filter and filterWithKey.
Fix imports
A little reorganizing of filterWithKey
implemented insertWithKey and insertWith

--HG--
extra : rebase_source : f023e606e25ed4a43417b24a09ce2bf10497354e
extra : amend_source : 4651bb5169452f93061b3d2e39340480b50cff54
extra : histedit_source : e23239ce3991c3f2152fb48445d37e86c4e6a810%2C180950865f61f834133223ba82549229c5173620
Fix 80-column issue
Make commit expectations clearer
Add a test for null

Adapted and simplified from a pull request by Ramses de Norre.
Fix deleteFindMin
Fix deleteFindMax
Small improvements to the test code.