(opt) add Optional::transform, allowing an arbitrary operation on an Optional

Add a method something like this to Optional:

    public <R> R transform(Function<Optional<T>, ? extends R> f) {
        return f.apply(this);
    }

This allows the caller to provide an arbitrary operation to be performed on "this" Optional, regardless of whether it's empty or present. The name "transform" follows precedent set by String::transform (JDK-8203442).