Migrate to Swift 6 language mode

[fwcd]

This bumps the required Swift version to Swift 6, which introduces data race safety via strict concurrency checking. Consequently, a larger refactoring is required.

Useful Tricks

Sendable Closures

When encounting an error like this:

182 | 
183 |                 Task {
    |                 `- error: task-isolated value of type '() async -> ()' passed as a strongly transferred parameter; later accesses could race

it has proven to be useful to mark the closure as @Sendable to get a better error message. After issues have been fixed, the annotation can be removed (for clarity/readability) since sendability can be inferred.

Note

In cases where @preconcurrency imports are used, the same trick may have to be applied again once the libraries have been updated if the annotation is removed, since the closures may implicitly become non-sendable again.

Shared Mutability

Classes with interior mutability are usually best modeled as actors.