Packer for `Vec` and `String` etc
For all methods index add a `offset`
```rust
use offset_vec::Offset;
let mut vec = vec![0, 1, 2, 3, 4];
let mut vec1 = vec.offset_mut(2);
assert_eq!(vec1, [2, 3, 4]);
assert_eq!(vec1[1], 3);
vec1[1] += 2;
assert_eq!(vec, [0, 1, 2, 5, 4]);
```
If there is no need for reallocation, consider [tailvec](https://github.com/A4-Tacks/tailvec-rs)