Move all Java interop to the end

[k4y3ff]

Currently, functions.clj has a "Java Interop" section, as well as an exercise that requires calling Java methods.

This is helpful for Java developers, but maybe half of the folks I've taught intro-level Clojure to have recent (or any) experience with Java; most come from Ruby or Python or PHP or something else entirely. In those cases, the Java references are distracting.

To that end, it would probably be better to just move all of the Java interop-related stuff into a new section, then stick that section towards the end of the workshop, just before "State".