quickchecking broader range of viable code

[shnewto]

The quickchecking crate, for property testing with fuzzed C headers could be extended to exercise a broader range of viable code.

Some examples of generated features that might provide value would be bitfields, C++, and other problematic code documented in issues tagged with I-bogus-codegen.

Corresponds to extending #970

The quickchecking crate lives in the tests directory here: https://github.com/rust-lang-nursery/rust-bindgen/tree/master/tests/quickchecking/