Switch from Leiningen to Clojure CLI, tools.build and kaocha

[devurandom]

[martinklepsch]

This seems reasonable, would you mind updating the contributing section in the README to match the Clojure CLI setup? In particular I'm not so sure about deploying to Clojars with this new setup — you probably have some sense of how that'll work?

If this is not yet set up maybe https://github.com/slipset/deps-deploy is a good option... I can set this up if you think it's the right move.