Add ring example

This example was translated from the Erlang ring benchmark written by Jiri Isa and optimized by Shun Shino:
* http://benchmarksgame.alioth.debian.org/u32/program.php?test=threadring&lang=hipe&id=1

This LFE version differs from the Erlang version in that it splits the code into more functions for increased readability.

This code is also now given as an example on the LFE docs site front page:

* http://lfe.github.io/