Make ActorQueue's tasks isolated to an actor's context

[dfed]

This PR is built on #3

This PR is similar to #6 but applied to the ActorQueue. However, rather than adding API, this PR replaces existing async and await API with methods where the tasks sent to the queue to execute on the designated actor's context.

The goals of this PR are:

1. Enable multiple calls to the sam actor within a single task without requiring multiple suspension points (await calls) within the `task
2. Ensure that await calls within a single task actually result in suspension

These goals are important because an ActorQueue's task allows other tasks to begin execution as soon as the task suspends (via an await). Since ActorQueues should only be utilized with a single actor instance, it made sense to replace the existing API rather than add new API – there's no good reason (that I can come up with) to use the the replaced APIs now that these new APIs exist.

All that said, I don't love this API change. ActorQueues are meant to be used as private implementation details of an actor, and as such the isolatedActor that will be sent to the ActorQueue's API is almost always going to be self. The big drawback of this change is that Swift doesn't allow parameters to a closure to be named self, so ActorQueue calls will likely end up looking like the following:

queue.async(on: self) { myself in
    myself.doSomething()
}

If someone were to try to ignore the parameter in the closure, they'd end up having to write an await:

queue.async(on: self) { _ in
    // This `task` is isolated to the parameter, which Swift doesn't know is the same as `self`.
    // The result is that Swift requires us to add an `await` here even though we're executing within `self`'s context.
    await self.doSomething()
}

I really like the goals I'm trying to achieve, but I don't love how the semantics play out. Curious to get your thoughts!

[dfed]

I'm tempted to add a new internal method called func allEnqueuedTasksSuspendedOrCompleted() async to make this test call-site read more clearly since we have a few of these.

[codecov]

Codecov Report

Merging #7 (fa286f8) into dfed--fifo-vs-actor (ea6ade9) will not change coverage.
The diff coverage is 100.00%.

@@                  Coverage Diff                  @@
##           dfed--fifo-vs-actor        #7   +/-   ##
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  Coverage               100.00%   100.00%           
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  Files                        2         2           
  Lines                       95        95           
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  Hits                        95        95           

Impacted Files	Coverage Δ
Sources/AsyncQueue/ActorQueue.swift	100.00% <100.00%> (ø)

[dfed]

Removing reviewers and bringing this PR back to drafts. Had some ideas I want to play with – I may be able to simplify call-sites make the async for-loop run on the target actor's context.

[dfed]

Ideally the ActorQueue would run entirely on its target actor's execution context, but that's not possible today given how Tasks and isolated parameters interact today.

I'm going to close this out for now, though I'll want to explore this path more in the future.