Add concurrency construct(s)

It is becoming increasingly common in high-performance computing to hear assertions that CS education programs need to integrate concurrency and parallelism patterns throughout the curriculum, including even the intro courses.

There is perhaps some merit to these assertions; imagine how much easier parallel computing would be if students learned from the very beginning how to identify and express dependency-free computation. Some argue that this mode of programming is actually more intuitive ("perform action X on every element in this collection" is often inherently dependency-free) and that we lose something by forcing everyone to learn a procedural paradigm first, linearizing the computation unnecessarily.

However, it's unclear how best to integrate concurrency into a language like Less-Java without overly complicating the language or introducing major safety issues. Perhaps it could mean adding something similar to [futures](https://en.wikipedia.org/wiki/Futures_and_promises), which are already implemented in Java and thus could be relatively easy to add to Less-Java. Perhaps it could involve adding annotations that the compiler converts to threaded code, like [OpenMP](https://en.wikipedia.org/wiki/OpenMP). Or maybe it's adding new language keywords like `spawn` / `sync` in [Cilk](https://en.wikipedia.org/wiki/Cilk). This is open future work.