quickhecking crate CLI

[shnewto]

The quickchecking crate, for property testing with fuzzed C headers, is a library at the moment. If instead it was a binary application some configuration could be exposed through a CLI.
(crate lives in the tests directory here: https://github.com/rust-lang-nursery/rust-bindgen/tree/master/tests/quickchecking)

Some configuration that may be worth exposing:

Number of tests to run

Each test on a fuzzed header takes about ~30 to run through the csmith-fuzzing/predicate.py script. In order to run in under ~1 minute, the default is to only generate and run 2 tests. It would be nice if that was easier to toggle.

Generate range

The range that quickcheck uses during generation is configurable. This range corresponds to things like arbitrary usize and arbitrary vector length. Currently this value is also defaulting to cater to a reasonable execution time. This number doesn't have to grow much for that execution time to increase significantly.

Provide generated headers for inspection

Default behavior is to generate temporary fuzzed headers in a directory decided by the TempDir crate as well as the quickchecking crate's tests directory. Providing/disabling the visibility of that code requires commenting/uncommenting some code in the fuzzed-c-headers.rs test file. It'd probably be nice to make that easier.

Corresponds to extending #970