Implement Equilibrium Index problem.

Not too happy with how the `filter_map` part turned out. Suggestions to improve are welcome!
I don't like that this closure has side efects... What do you think about an imperative approach? I find it easier to read.

```
let mut right = v.iter().map(|&x| x).sum();
let mut left: T = Zero::zero();
let mut result = vec![];

for (i, &el) in v.iter().enumerate() {
    right = right - el;
    if left == right {
        result.push(i)
    }
    left = left + el;
}
result
```