Create a future from a legacy callback, e.g. `(LPLinkMetadata?, Error?) -> Void`

Future+Legacy.swift

import Combine

extension Future {
    static func legacy(_ attemptToFulfill: @escaping (@escaping (Output?, Failure?) -> Void) -> Void) -> Future {
        Future { completion in
            attemptToFulfill { output, error in
                switch (output, error) {
                case (nil, let error?):
                    completion(.failure(error))
                case (let output?, nil):
                    completion(.success(output))
                case (_?, _?):
                    preconditionFailure("Future.fromCompletionHandler unexpectedly received both non-nil `output` and non-nil `error`")
                case (nil, nil):
                    preconditionFailure("Future.fromCompletionHandler unexpectedly received both nil `output` and nil `error`")
                }
            }
        }
    }
}

It's probably better to just resume with https://gist.github.com/JadenGeller/a7f5a96bbca42fdce524d80fe1231c4f