Unexpected animator state in completion block

[juliensagot]

Hi 👋

The state of an animator is being updated after calling its completion block, resulting in being incorrect in said completion block.
This is major issue when you want to share a completion block with multiple animators with different settling times, and execute some logic after all animators have completed.

For example:

let firstAnimator = SpringAnimator<CGFloat>(...)
let secondAnimator = SpringAnimator<CGRect>(...)

let sharedCompletion = {
    guard case .ended = firstAnimator.state, case .ended = secondAnimator.state  // Will never be satisfied
    else { return }
    print("Both animations are finished.")
}

firstAnimator.completion = { completion in
    guard case .finished = completion else { return }
    // (firstAnimator.state == .ended) → false 😭
    sharedCompletion()
}

secondAnimator.completion = { completion in
    guard case .finished = completion else { return }
    // (secondAnimator.state == .ended) → false 😭
    sharedCompletion()
}

I understand that you don't accept contributions yet so I haven't opened a pull request, but if you change your mind, I don't mind opening one!
I forked the project and fixed the issue in the meantime.

Thanks again for this delightful animation engine, I love it!