Include TCA Swift Concurrency changes from release 0.39.0

[mluisbrown]

TCA just released version 0.39.0 which includes all the Swift Concurrency changes, and fundamentally changes many internals of the library. For example, Effect no longer conforms to Publisher but instead holds on to a Publisher internally.

The changes encourage moving away from Combine based effects to effects that use Swift Concurrency using Effect.task.

I've been able to maintain this fork in sync with TCA up to the last commit prior to the merge of the Swift Concurrency PR, however I feel that trying to update the fork to include the Swift Concurrency changes would be a huge task, and I don't have the necessary time to dedicate to it.

There's also the question of how useful this fork will continue to be. The objective for @mbrandonw and @stephencelis with TCA is clearly (and rightly) to be able to remove Combine completely from TCA sooner or later, at which point the need for a Reactive Swift fork really ceases to exist. Even now, this fork has a minimum deployment target of iOS 13 so the only use cases for it are:

• existing projects using it which haven't yet migrated to Combine TCA
• projects that don't want to use Combine as it's not open source
• projects using non-Apple OSes like Linux and Android

If anyone is interested in trying to migrate this fork to the new changes, I'd be happy to help out where I can.

[mluisbrown]

Thanks to @p4checo for taking on this huge task. The fork has now been updated to include the upstream changes from 0.39.0 and more, so this issue has been resolved!

Fixed by: #58