SE-0340: Unavailable From Async Attribute

[tclementdev]

@etcwilde

Blocking issues would also need their own way to indicate things that shouldn't be thrown into the general cooperative pool, which was another point brought up in the initial pitch discussions.

@rokhinip on this thread says:

One other thing I'd like to highlight is that not all blocking is bad. If you are blocking on IO, or using an os_unfair_lock for data synchronization purposes in a tight critical section, the blocking here is temporary and therefore, fine. The effect is that you may have reduced throughput on the cooperative thread pool, but you will still be able to make forward progress.

Then why insist this much on marking locks as unavailable from async? I understand that using locks is not recommended, but having the compiler throw an error on them does not seem right when their usage is deemed "fine".