Emitted elements doesn't match Combine's publisher(for:)

[simba909]

Comparing to the Combine publisher(for:) API:

// where player is an AVPlayer
let cancellable = player.publisher(for: \.rate)
  .sink { rate in
    // "rate" here isn't optional, just like the property on AVPlayer
  }

// this library:
for element in player.sequence(for: \.rate) {
  // element.newValue here is optional :/
  // element also has an oldValue property,
  // which the Combine counterpart doesn't surface
}