Quickcheck and test coverage

[japaric]

Quickcheck generates integer values in a small range around zero ([-100, 100]) (cf BurntSushi/quickcheck#119). These values are not enough to tests all the branches of some intrinsics implementation. We need better control over the generated values; here's are some possible solutions of the top of my head:

• Synthesize integer values from two halves. That's is use (u32, u32) as quickcheck argument and then use u64::from_parts(some_u32, another_u32) in the actual test.
• Create a newtype, e.g. U64(u64), and implement the Arbitrary trait for it, but have the implementation generate values in the [MIN, MAX] range.