Implement Equilibrium Index problem.

Not too happy with how the `filter_map` part turned out. Suggestions to improve are welcome!
just FYI: it mentions in https://github.com/rust-lang/rust/pull/11449 that a += b used to be desugared to a = a + b, but now this has been removed (although I could not find more explanations on when and why, it seems by looking at the related issues to have something to do with creating traits that allow you to customize assignment)