Full-program doc examples? #57

The guide and reference describe Hackett's features in isolation at a REPL, but there aren't any examples of a full Hackett module in the docs. The example code in the README is the sort of thing I'm thinking of. A small (<= 10 lines) complete module might be especially helpful on the very front page of the docs so users can quickly get a sense of what Hackett code looks like.