Add bias tests

`Data.Map` and `Data.Set`, and probably also the `Int` versions, document certain bias properties that don't seem to be checked by the test suite (except for one incomplete check that I just added). For example, we expect that if sets `xs` and `ys` share an element `z`, then `union xs ys` will take the copy from `xs`. For maps, I believe the values may be tested fine, but we need to test key bias. I've added an `OddEq` type whose `Eq` instance is not the usual equality; this should be good for testing these properties.