Actors in coswift

**Is your feature request related to a problem? Please describe.**
It's currently not possible to implement coroutine actors using the coswift library in Swift code.

**Describe the solution you'd like**
It would be helpful for `co_actor()` and related functions and classes in coobjc to be implemented in coswift for usage in Swift code.

**Describe alternatives you've considered**
I've tried using the functions from the coobjc Objective-C API and there are a number of issues with getting it to work nicely in Swift. For example, `COActorChan` can't be iterated in a for-loop. And `COActorCompletable` is a `COPromise`rather than the coswift equivalent `Promise`. Also, importing the `` API in the Swift bridging header introduces ambiguous function definitions such as `co_delay()`.

Start by comparing the coswift API with the coobjc implementations of co_actor() and its related functions and classes. Check how COActorChan, COActorCompletable, Promise, and the co_delay() naming conflict are exposed, then define completion as equivalent actor APIs that work naturally from Swift without the reported bridging issues.

Written by the indexing model from the issue text.