This ended up being really straightforward! I discovered a couple of
behavior differences around the presence of build scripts -- it would be
interesting to report this upstream.

Upstream cargo had some fixes that I ended up having to pull in, hence the
git dependency.

Also include a failed attempt to allow setting a custom
home dir (sorry!)

This works by:
* setting up a real, fairly complex fixture
* generating random queries using proptest
* running them through cargo and guppy
* ensuring that they produce the same results

Not only does this give us high confidence that cargo
and guppy produce the same results, it also gets us
most of the way to facebookarchive#83.

Most of the work in this commit is simply setting up
all the necessary test infrastructure. We piggy-back on
existing proptest infrastructure as much as possible.

Do this in a separate job to allow for greater parallelization.

Make proptests more interesting by generating random platforms for tests.