added lisp lists example

[BartMassey]

I added a reasonably comprehensive example of crate use to gc-arena. It shows some working with LISP-style lists. Hopefully this will help folks figure out how to get started. I haven't yet mentioned the example in any of the docs, but it might be a good idea.

A smaller example in the Rustdoc and README for gc-arena would also be helpful. I can provide that if desired.