Implement Index and IndexMut traits

[mikwielgus]

It's useful to have Index and IndexMut traits implemented if you want to have code that is generic over indexable containers, like this:

fn increment<C>(container: &mut C, index: usize)
where
    C: std::ops::IndexMut<usize>,
    C::Output: std::ops::AddAssign<i32>,
{
    container[index] += 1;
}

Having these traits is also consistent with Vec, which also implements them.

I have added two tests to test these traits.

[alejandro-vaz]

wait... weren't they implemented already??

[alejandro-vaz]

rustfmt is complaining about styling, simply run cargo fmt --all

[alejandro-vaz]

I thought we had Index and IndexMut somewhere wtf

[mikwielgus]

rustfmt is complaining about styling, simply run cargo fmt --all

Done. (I actually also had to point it to nightly Rust's rustfmt to work)

[alejandro-vaz]

thanks @mikwielgus, not sure how we missed this

[mikwielgus]

I thought we had Index and IndexMut somewhere wtf

Indexing and mutation of an indexed element obviously still worked without Index and IndexMut because of Deref coercion into slice, which actually does have Index and IndexMut implemented. This is why these traits are easy to miss for vec-like collections. I had the same issue with thin-vec.