Add materializeResults operator

[ra1028]

This operator is useful when we want to handle Result without to complete stream.
It has semantics similar to dematerialize.
And dematerializeResults which works in inverse is also added.

Use case:

// This stream doesn't complete even if an error sent.
viewModel.reactive.date.producer
    .take(duringLifetimeOf: self)
    .flatMap(.latest) { date in
        repository.getPrograms(on: date)
            .materializeResults()
    }
    .startWithValues { result in
        switch result {
        case .success(let programs)
            print(programs)

        case .failure(let error)
            print(error)
    }

Checklist

• Updated CHANGELOG.md.

[mdiep]

Awesome! 👍🏻

I've wanted this for a while, but I had a hard time deciding on a name. materializeResults and dematerializeResults seem like good choices.