Allow index of split_off to equal length of vector

[bbc2]

Allows:

    let mut left = vector![0, 1, 2];
    let right = left.split_off(3);
    assert_eq!(left, vector![0, 1, 2]);
    assert_eq!(right, vector![]);

This makes it more similar to the primitive slice split_at from the standard library (see https://doc.rust-lang.org/std/primitive.slice.html#method.split_at).

[bodil]

This makes a lot of sense. 👍