Quickcheck: Investigate always testing special cases

[japaric]

Right now we test the intrinsics by feeding them random input arguments. Given that we have full control of the Arbitrary implementation perhaps we can tweak the implementation to always generates special cases at the beginning. In other words, change the generated inputs from:

Today: e.g. i32 inputs: <rand>, <rand>, <rand>, ...
To: i32::MIN, 0, i32::MAX, <rand>, <rand>, ...

[japaric]

With #49, we now are testing edge cases. #48 will test edge cases on floats.