Strict concurrency checks

[Gerzer]

Swift 5.7 has an option for “strict concurrency checking”. When it’s enabled, the compiler tries to prove that low-level data races are impossible. If that can’t be proven (or if the checks are locally bypassed in the source code), then compilation fails. Swift 6 will make some of these checks mandatory.

We should ensure that our code passes strict concurrency checks. This will be difficult, not least because many system frameworks haven’t yet been updated to support proper actor isolation.