Include an queue-backed actor example

[mattmassicotte]

This adds both disucssion and an example of a queue-backed actor.

However, I've just discovered that the asUnownedSerialExecutor API isn't available in Xcode 16 beta 1. So, I'm just going to leave this in draft form until a new beta is released.

[FranzBusch]

However, I've just discovered that the asUnownedSerialExecutor API isn't available in Xcode 16 beta 1. So, I'm just going to leave this in draft form until a new beta is released.

DispatchSerialQueue adopted SerialTaskExecutor on macOS 14.0 so this should already be available in Xcode 15. How did you test this? Importantly, there is currently a compiler bug with availability so you have to check that the minimum platform version for your target is macOS 14.0 otherwise you won't be able to access the conformance to SerialTaskExecutor of the queue.

[mattmassicotte]

@FranzBusch I think that bug is exactly what I'm experiencing!

If I increase the package platform minimum to macOS 14, this code works without any availability guards required. But, with just the @available and #available I'm currently using, it does not.

[ktoso]

Sounds weird? @available should work fine -- please file a bug if you can reproduce it

[FranzBusch]

Tracked internally in rdar://116684282

[DougGregor]

I came looking for exactly this PR, thank you! I'm going to look into the compiler bug that's causing problems with @available here, but I'd urge you not to block merging this pull request on our resolution of that bug.

[mattmassicotte]

@DougGregor thank you so much for checking in on this. Sorry it took me so long to follow up. I've commented out the offending code to keep the examples building.