Seeing if I can make a rust implementation of `flip` that is reminiscent of Haskell's (http://hackage.haskell.org/package/base-4.6.0.1/docs/Prelude.html#v:flip)

flip.rs

fn flip<T>(f: ~fn(a: T, b: T)) -> ~fn(a: T, b: T) {
  |a, b| { f(b, a) }
}

fn hello_world(hello: &str, world: &str) {
  println!("{:s}, {:s}!", hello, world)
}

#[test]
fn test_flip() {
  flip(hello_world)("hello", "world");
}