ValueObservation.compactMap

[groue]

This PR introduces ValueObservation.compactMap, which notifies the non-nil results of a transformation applied to the notified elements of a base ValueObservation:

extension ValueObservation where Reducer: ValueReducer {
    func compactMap<T>(_ transform: @escaping (Reducer.Value) -> T?)
        -> ValueObservation<CompactMapValueReducer<Reducer, T>>
}