rfc 2033 example

ex.rs

let mut v = [-5i32, 4, 1, -3, 2];
v.sort_unstable();
assert!(v == [-5, -3, 1, 2, 4]);
v.sort_unstable_by(|a, b| b.cmp(a));
assert!(v == [4, 2, 1, -3, -5]);
v.sort_unstable_by_key(|k| k.abs());
assert!(v == [1, 2, -3, 4, -5]);