Main example for HashSet.

Example code for HashSet, similar to the [HashMap example](http://doc.rust-lang.org/std/collections/hashmap/struct.HashMap.html).
I see. It was a bit cumbersome to convert a fixed size list to set, it would have been nice to either have a `.move_iter()` or a `vec!` like construct. It wasn't nice to  use two different approaches though.