Ati, ergonomic indexing of Vec

The ati crate introduces the At trait, and implements it for Vec. The At trait adds a at and at_mut method, which allows a Vec to be indexed by u8, u16, u32, u64, u128, as well as i8, i16, i32, i64, i128, isize.

Negative indexes allows for indexing in the reverse direction, exactly how the Javascript at function works, or Python indexing.

Examples

use ati::At;

fn main() {
    let mut v = vec![1,2,3];

    assert_eq!(1, *v.at(0u8));
    assert_eq!(3, *v.at(-1u128));
    
    *v.at_mut(-1) = 5;

    assert_eq!(&[1, 2, 5], &v[..]);
}