QCheck-fun: Fun examples for exercising QCheck's function generator

This is a collection of examples from

• JSVerify https://github.com/jsverify/jsverify
• Koen Claessen's Haskell'12 pearl "Shrinking and Showing Functions"
• Ulf Norell's talk "Generating random functions"

Note: latest commit tests QCheck 6448cc4cc1bdf5b0ee169143fadef3004557638b

Some observations:

• shrinking is much more effective

• shrinking may occasionally take very long

• some things could potentially be shrunk even further, e.g., the 7414 here:

  Test map filter failed (40 shrink steps):

  ({_ -> 0}, {7414 -> false; _ -> true}, [7414])

Old observations:

• shrinking does not work as well over functions (counterexamples are even greater with int generator)
• the printed functions are partial (Claessen in Haskell and Norell's eqc_fun in Erlang prints them with a final _ catch all case)
• can one express Norell's prop_losing as a recursive invocation of QCheck_runner.run_tests?