Add test suite

[treeowl]

The easiest way to get started will probably be to build a clone of most of Data.Map using

newtype Map k v = Map (DMap (Const k) (Const v))

and then just copying large chunks of the Data.Map test suite out of containers.

The same approach should also help get benchmarks started.

[Wizek]

Would be nice, looking at tests for usage examples might have alleviated my issue with #16.