[Optics] Optional getter

[serras]

PR #2588 has shown that there's a missing element in the optics hierarchy: folds focus on 0..N, getters focus on exactly 1, so we are missing something which focuses on 0..1. This is OptionalGetter, the "get" part of Optional and Prism. In other libraries this is called an AffineFold. However, I find that name very obscure. Feel free to suggest changes.

In particular, the operation of filtering is one such OptionalGetter. The implementation is a copy of the one provided by @lenguyenthanh in #2588 (thank you for that!).

A technical note: you may notice that it is not the case that Getter<S, A> extends OptionalGetter<S, A>. This is because the right way to describe the relation is that Getter<S, A> is a POptionalGetter<S, T, A> for any choice of T (since it never returns it). Alas, Kotlin does not allow us to declare that relationship. One way to handle this would be to use something like OptionalGetter<S, Nothing, A>, but unfortunately the T must be invariant :(

[nomisRev]

Looks good to me, a small mistake in the title of the markdown doc.