Map by KeyPath

[RomanPodymov]

Hello.
Thank you for BrightFutures.
In this pull request I added func map<U>(_ context: @escaping ExecutionContext, keyPath: KeyPath<Value.Value, U>) -> Future<U, Value.Error>. It is similar to map that you already have, but accepts KeyPath instead of a closure.

[Thomvis]

Hi, thanks for contributing to BrightFutures!

I like your contribution and I'd love to merge it after discussing one thought I had while going through the changes. Do you think you could leverage the closure-based map implementation to simplify the new map function? So instead of calling into onComplete, call map with a closure that uses the key path. That way we're guaranteed to match the behavior of map.

Let me know what you think!

[RomanPodymov]

Hello @Thomvis
I agree with you. Also set !swift(>=5.2) for all new functions because Swift 5.2 provides Key Path Expressions as Functions. testMapByKeyPath is available for all Swift versions.

[Thomvis]

Thanks for making these changes. I've merged the PR! :)