Update the project to address concurrency failures for 6.2 build modes.

[phausler]

When updating to the latest language mode there are a ton of warnings that became errors around Swift Concurrency. This addresses all of the errors so far (and caught a subtile compiler bug for sendable types within the buffer algorithm).

This still has a few warnings around Sendable meta types.

e.g.

AsyncChunksOfCountOrSignalSequence.swift:169:33: warning: capture of non-Sendable type 'Collected.Type' in an isolated closure [#SendableMetatypes]
167 |     return Iterator(
168 |       iterator: merge(
169 |         chain(base.map { Either.element($0) }, [.terminal].async),
    |                                 `- warning: capture of non-Sendable type 'Collected.Type' in an isolated closure [#SendableMetatypes]
170 |         signal.map { _ in Either.signal }
171 |       ).makeAsyncIterator(),

These are relatively prolific but likely need to be addressed by restricting around a where clause requiring SendableMetatype https://docs.swift.org/compiler/documentation/diagnostics/sendable-metatypes/

[phausler]

The 5.9 break is kind-of expected with the 6.2 tools version; but that was just put in to verify the 6.2 language mode worked as expected - from my understanding that can be reverted back similarly to how the Package@swift-5.7.swift works

[FranzBusch]

Can we leave a comment here why the unsafe transfer is needed and why it is sound?