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

try to get around the timeout problem on slow CircleCI builders