Add bias tests

[treeowl]

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.

[sjakobi]

Data.Semigroup.Arg might also be useful in this context.