Generate fake values with full constructor coverage

The idea behind Fake.Cover is that although exhaustive testing is highly exponential, you can cover a large portion of the likely problem cases by exercising all the constructors of a data type and associated fields. This approach only requires a sub-exponential number of cases--far fewer than what you need for the exhaustive approach. The number of test cases needed to ensure that you have full coverage of all the constructors is given by the following relations for product and sum types:

numCases (a, b) = max (numCases a) (numCases b)

numCases (Either a b) = numCases a + numCases b

See the test suite for examples of how many values are generated for different data types.