This avoids needless build churn and potentially destabilizing changes from
upstream (though the latter does not seem likely since upstream changes should
not affect packages in our resolver, only newer resolvers).

Also change a Maybe to an Either so we can stop printing an error inside the
ristretto code and just propagate it up to the caller.

None of the FFI values escape from `ristretto` so all of the FFI IO should be
safe to do with `unsafePerformIO`.

Construct a better Nix store cache key for CircleCI

It's total amateur hour but that's what I am so who is surprised?

Remove the test suite

Add the Ristretto dependency