Introduce quick check and use a modern unit test framework

[yoriyuki]

No description provided.

[rgrinberg]

I nominate https://github.com/mirage/alcotest for a modern alternative.

[rgrinberg]

Another useful alternative here is https://github.com/janestreet/ppx_inline_test

Writing tests this way is quite nice and will be natively supported in jbuilder/dune very soon. The only downside is that it introduces a ppx dependency for building Camomile. I don't think that's such a big deal though so I'd still recommend it.