Add Monoid instance for Set k.
Uncomment mapMonotonic from Set and add tests.

Relax type signature for mapKeysMonotonic.
A few substitutions with equivalent expressions.
Add Monoid instance for CritBit k v.
Make use of -XRecordWildCards.
Merge pull request #80 from archblob/XRecordWildCards

Make use of -XRecordWildCards.
Save 80ns on lookup benchmarks

Based on a patch from archblob <fcsernik@gmail.com>.

--HG--
extra : amend_source : c69600f06aaac6c8a963b9d63e7cca41e49fd489
Merge pull request #76 from archblob/Set_Monoid

Add monoid instance for Set. Uncomment and add test for mapMonotonic.