Adopt Swift 6 and audit for Sendable

[parkera]

This patch:

• Adopts Swift 6 mode for the libraries, executables, and tests
• Updates all types for Sendable annotation, and adds a flag to require all future types to do so as well
• Syncs with Darwin Foundation for noasync functions
• Addresses all Sendable warnings.
  • In many cases this is done by using nonisolated(unsafe). This should be considered a short term workaround and not a design pattern. In the future, adding these will be disfavored and rightfully proving to the compiler that it is safe will be preferred. However, it's not practical to do all of that in one shot.
  • Many warnings were also addressed by use of new Synchronization types and more annotations.
• Addresses all remaining warnings of all other types.

[parkera]

I'm not sure how reviewable this is. It is the result of repeated iteration and small fixes.