First beta release

• compiles in Swift 6 language mode (requires Swift 6.2 or greater)
• datagram client and server examples now support domain sockets
• use Async[Throwing]Stream in lieu of Async[Throwing]Channel: this should improve performance, but the API consumer is now responsible for managing back-pressure
• remove global actor: IORing instances are now independent actors, and helper classes are constrained to their isolation domain.
• avoid name conflict with upstream IORing type in SystemPackage (note though, at present, IORingSwift still uses liburing; this is unlikely to change in the near future)
• Socket API now provides API for getting and setting arbitrary socket options
• elide some unnecessary Task creations
• fix accidental swapping of fixedBuffers.size and fixedBuffers.count
• more tests, various other small fixes