Quickchecking crate CLI

Prior to this commit the quickchecking crate used for generating proprty
tests
for bindgen was a [lib] target and had configurations that required
commenting/uncommenting code to enable/disable. This meant it was
inconvienent/prohibitive to configure the property tests on a per-run
basis.
This commit reorganizes the `quickchecking` crate to provide both [lib]
and
[[bin]] targets in order to expose those configurations through a CLI.

The configurations that are exposed through the [[bin]] target's CLI
are:
* Count/number of tests to run.
* Directory to provide fuzzed headers
* Generation range corresponding to the range quickcheck uses to
* generate arbitrary.

__Usage from the__ `tests/quickchecking` __directory__
```bash
quickchecking 0.2.0
Bindgen property tests with quickcheck. Generate random valid C code and
pass it to the csmith/predicate.py script

USAGE:
    quickchecking [OPTIONS]

FLAGS:
    -h, --help       Prints help information
    -V, --version    Prints version information

OPTIONS:
    -c, --count <COUNT>    Count / number of tests to run. Running a
fuzzed header through the predicate.py script can
                           take a long time, especially if the
generation range is large. Increase this number if you're
                           willing to wait a while. [default: 2]
    -p, --path <PATH>      Optional. Preserve generated headers for
inspection, provide directory path for header
                           output. [default: None]
    -r, --range <RANGE>    Sets the range quickcheck uses during
generation. Corresponds to things like arbitrary usize
                           and arbitrary vector length. This number
doesn't have to grow much for that execution time to
                           increase significantly. [default: 32]
```

Because the actual work of running the property tests moved to the
[[bin]]
target, rather than duplicate that code in the `quickchecking` crate's
tests
directory, some actual (very basic) tests for the `quickchecking` crate
were
added.

*Note: I'm not attached to any of the option flags, if there are better
characters/words for any of the options I've exposed I'll be happy to
revise!

Thanks for taking a look, looking forward to feedback!

Closes #1168

r? @fitzgen

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