Temporary Annex

Allows pushing to a Vec (or any other type implementing Annexable) with cleanup once result goes out of scope.

e.g.

let mut vec1 = vec![1, 2, 3];
assert_eq!(vec1, [1, 2, 3]);
{
    let new_vec_ref = vec1.push_annex(4);
    assert_eq!(*new_vec_ref, [1, 2, 3, 4]);
}
assert_eq!(vec1, [1, 2, 3]);

This has the effect of a immutable structure but uses the same underlying allocation (no cloning or additional allocations).