Permutations of [1 element] should not be empty

[wackywendell]

The permutations of [x] should be the list [[x]].

Additionally, I would also expect the permutations of [] to be [[]] (as it is in python, which also matches 0! = 1! = 1).

Currently, the permutations function (std::slice::ImmutableCloneableVector::permutations) returns [] in both cases.

Example code:

fn main(){
    let clists = [~[], ~['x'], ~['x','y']];

    for clist in clists.iter(){
        let perms : Vec<~[char]> = clist.permutations().collect();
        println!("Permutations of {}: {}", clist, perms);
    }
}

Prints out

Permutations of []: []
Permutations of [x]: []
Permutations of [x, y]: [[x, y], [y, x]]

Instead of the expected

Permutations of []: [[]]
Permutations of [x]: [[x]]
Permutations of [x, y]: [[x, y], [y, x]]