examples using core.async

Something that could show off the capabilities of Clojure to learners is core.async, which would give a taste of how Clojure makes concurrency easier.

It should be possible to have multiple turtles acting independently in a canvas, similar Rich Hickey's ants demo (https://youtu.be/hb3rurFxrZ8?t=49m50s), although we only have atoms (no refs) in clojure-turtle because it is preferable to have the same code work the same in CLJS and Java Clojure.

This concept is very much like [NetLogo](https://ccl.northwestern.edu/netlogo/), for which there is already a set of materials built up around the concept.

Regardless of whether we attempt to create teaching materials that correspond to NetLogo, at least NetLogo would contain some nifty examples that are worth mimicking.